use instead:

@media only screen and (max-width: 767px) {
  .responsive #top .av-special-heading .av-subheading p {
    font-size: 36px !important;
    line-height: 1.5em !important;
  }
}

because you have set in your css:

@media only screen and (max-width: 767px) {
  body, p, .entry-content {
    font-size: 18px !important;
    line-height: 1.6 !important;
  }
}

it is necessary to do so aswell with !important

have a look at : https://webers-testseite.de/bemopriv/
the heading had to be styled for responsive case – but I didn’t feel like doing that anymore.

the layout is based on a grid-row element

this is the enfold shortcode of the grid-row:

[av_layout_row min_height_percent='percent' min_height_pc='60' min_height='0' border='' fold_type='' fold_height='' fold_more='Read more' fold_less='Read less' fold_text_style='' fold_btn_align='' color='main_color' fold_overlay_color='' fold_text_color='' fold_btn_color='theme-color' fold_btn_bg_color='' fold_btn_font_color='' size-btn-text='' av-desktop-font-size-btn-text='' av-medium-font-size-btn-text='' av-small-font-size-btn-text='' av-mini-font-size-btn-text='' fold_timer='' z_index_fold='' mobile='av-flex-cells' mobile_breaking='' mobile_column_order='' id='' custom_class='full-bg-image' template_class='' aria_label='' av_element_hidden_in_editor='0' av_uid='av-m6f9v8gq' sc_version='1.0']
[av_cell_three_fourth vertical_align='middle' padding=',,,' av-desktop-padding=',,,80px' av-medium-padding=',,,80px' av-small-padding='3' av-small-padding_sync='true' av-mini-padding='20' av-mini-padding_sync='true' background='bg_color' background_color='' background_gradient_direction='vertical' background_gradient_color1='#000000' background_gradient_color2='#ffffff' background_gradient_color3='' src='' attachment='' attachment_size='' background_attachment='scroll' background_position='top left' background_repeat='no-repeat' link='' link_dynamic='' linktarget='' title_attr='' link_hover='' mobile_display='' mobile_col_pos='0' custom_class='' template_class='' av_uid='av-m6f9ukqb' sc_version='1.0']

[av_four_fifth first min_height='' vertical_alignment='av-align-top' space='' row_boxshadow_width='10' row_boxshadow_color='' margin='0px' margin_sync='true' av-desktop-margin='' av-desktop-margin_sync='true' av-medium-margin='' av-medium-margin_sync='true' av-small-margin='' av-small-margin_sync='true' av-mini-margin='' av-mini-margin_sync='true' mobile_breaking='' mobile_column_order='' border='1' border_style='solid' border_color='#000000' radius='15' radius_sync='true' min_col_height='' padding='30' padding_sync='true' av-desktop-padding='' av-desktop-padding_sync='true' av-medium-padding='' av-medium-padding_sync='true' av-small-padding='' av-small-padding_sync='true' av-mini-padding='' av-mini-padding_sync='true' svg_div_top='' svg_div_top_color='#333333' svg_div_top_width='100' svg_div_top_height='50' svg_div_top_max_height='none' svg_div_top_opacity='' svg_div_bottom='' svg_div_bottom_color='#333333' svg_div_bottom_width='100' svg_div_bottom_height='50' svg_div_bottom_max_height='none' svg_div_bottom_opacity='' fold_type='' fold_height='' fold_more='Read more' fold_less='Read less' fold_text_style='' fold_btn_align='' column_boxshadow_width='10' column_boxshadow_color='' background='bg_color' background_color='rgba(0,0,0,0.5)' background_gradient_direction='vertical' background_gradient_color1='#000000' background_gradient_color2='#ffffff' background_gradient_color3='' src='' attachment='' attachment_size='' src_dynamic='' background_position='top left' background_repeat='no-repeat' highlight_size='1.1' fold_overlay_color='' fold_text_color='' fold_btn_color='theme-color' fold_btn_bg_color='' fold_btn_font_color='' size-btn-text='' av-desktop-font-size-btn-text='' av-medium-font-size-btn-text='' av-small-font-size-btn-text='' av-mini-font-size-btn-text='' animation='' animation_duration='' animation_custom_bg_color='' animation_z_index_curtain='100' parallax_parallax='' parallax_parallax_speed='' av-desktop-parallax_parallax='' av-desktop-parallax_parallax_speed='' av-medium-parallax_parallax='' av-medium-parallax_parallax_speed='' av-small-parallax_parallax='' av-small-parallax_parallax_speed='' av-mini-parallax_parallax='' av-mini-parallax_parallax_speed='' fold_timer='' z_index_fold='' css_position='' css_position_location=',,,' css_position_z_index='' av-desktop-css_position='' av-desktop-css_position_location=',,,' av-desktop-css_position_z_index='' av-medium-css_position='' av-medium-css_position_location=',,,' av-medium-css_position_z_index='' av-small-css_position='' av-small-css_position_location=',,,' av-small-css_position_z_index='' av-mini-css_position='' av-mini-css_position_location=',,,' av-mini-css_position_z_index='' link='' link_dynamic='' linktarget='' link_hover='' title_attr='' alt_attr='' mobile_display='' mobile_col_pos='0' id='' custom_class='' template_class='' aria_label='' element_template='' one_element_template='' av_uid='av-t574on' sc_version='1.0']

[av_heading heading='FREEZE FLAT™' tag='h1' style='blockquote modern-quote' subheading_active='subheading_above' show_icon='' icon='ue800' font='entypo-fontello' size='' av-desktop-font-size-title='48' av-medium-font-size-title='42' av-small-font-size-title='36' av-mini-font-size-title='28' subheading_size='' av-desktop-font-size='36' av-medium-font-size='28' av-small-font-size='24' av-mini-font-size='20' icon_size='' av-desktop-font-size-1='' av-medium-font-size-1='' av-small-font-size-1='' av-mini-font-size-1='' color='custom-color-heading' custom_font='#ffffff' subheading_color='#ffffff' seperator_color='' icon_color='' margin='' margin_sync='true' av-desktop-margin='' av-desktop-margin_sync='true' av-medium-margin='' av-medium-margin_sync='true' av-small-margin='' av-small-margin_sync='true' av-mini-margin='' av-mini-margin_sync='true' headline_padding='' headline_padding_sync='true' av-desktop-headline_padding='' av-desktop-headline_padding_sync='true' av-medium-headline_padding='' av-medium-headline_padding_sync='true' av-small-headline_padding='' av-small-headline_padding_sync='true' av-mini-headline_padding='' av-mini-headline_padding_sync='true' padding='10' av-desktop-padding='' av-medium-padding='' av-small-padding='' av-mini-padding='' icon_padding='10' av-desktop-icon_padding='' av-medium-icon_padding='' av-small-icon_padding='' av-mini-icon_padding='' link='' link_dynamic='' link_target='' title_attr='' id='' custom_class='iceglas' template_class='' element_template='' one_element_template='' av_uid='av-m6e7xj5g' sc_version='1.0' admin_preview_bg='']
The Original
[/av_heading]

[/av_four_fifth][/av_cell_three_fourth][av_cell_one_fourth vertical_align='bottom' padding='' padding_sync='true' av-desktop-padding='' av-desktop-padding_sync='true' av-medium-padding='' av-medium-padding_sync='true' av-small-padding='' av-small-padding_sync='true' av-mini-padding='' av-mini-padding_sync='true' background='bg_color' background_color='' background_gradient_direction='vertical' background_gradient_color1='#000000' background_gradient_color2='#ffffff' background_gradient_color3='' src='' attachment='' attachment_size='' background_attachment='scroll' background_position='top left' background_repeat='no-repeat' link='' link_dynamic='' linktarget='' title_attr='' link_hover='' mobile_display='' mobile_col_pos='0' custom_class='' template_class='' av_uid='av-u4sunr' sc_version='1.0']

[av_video src='https://www.youtube.com/watch?v=W73GdyDSHuU' mobile_image='https://webers-testseite.de/wp-content/uploads/dynamic_avia/avia_video_thumbnails/youtube/W73GdyDSHuU/W73GdyDSHuU.jpg' attachment='49853' attachment_size='full' video_autoplay_enabled='aviaTBvideo_autoplay_enabled' html5_fullscreen='aviaTBhtml5_fullscreen' format='16-9' width='16' height='9' conditional_play='' id='' custom_class='' template_class='' element_template='' one_element_template='' av_uid='av-m6f9u95o' sc_version='1.0']

[/av_cell_one_fourth]
[/av_layout_row]

besides the possibility to set it directly in the element itself:
(here an example with screen-width less than a given value)

you can manage it by quick css too:
(you only missed to select the p tag inside your selector)

@media only screen and (max-width: 767px) {
  #top .av-special-heading .av-subheading p {
    font-size: 24px
  }
}

yes – and your mentioned code above : a.cart_dropdown_link leads me to the wrong direction.
But better is to post the link to that page – so i can inspect it there. – is it the priver page?
(f.e. the demo page Enfold Shop does not have that account login icon inside main nav. it is on header_top.)

#top #header .av-main-nav  #menu-item-wc-account-icon > a {
  color: #f19809;
  font-size: 28px;
}

or if you like to have a hover effect on this:

#top #header .av-main-nav  #menu-item-wc-account-icon > a {
  color: #f19809;
  font-size: 28px;
  transition: color 0.7s ease;
}

#top #header .av-main-nav  #menu-item-wc-account-icon > a:hover {
  color: #1c60c7;
}

the data-av_icon is on the following span tag:

#top a.cart_dropdown_link {
  right: 15px;
}

#top a.cart_dropdown_link span[data-av_icon] {
  font-size: 28px;
  color: var(--enfold-header-color-color);
}

/* === to influence the counter === */
#top a.cart_dropdown_link span.av-cart-counter {
  height: 20px;
  left: 64%;
  font-size: 18px;
  line-height: 20px;
  border-radius: 50%;
  background: var(--enfold-header-color-primary);
  color: var(--enfold-header-color-constant-font);
  margin: -22px 0 0 2px;
  padding: 2px 6px;
}

i inserted above the default colors – change them to your needs