Thanks, I guess I fixed my css by coping each code one by one back in.
On Desktop everything looks good again.
But the button on the mobile slider just won’t change.
Here is my current full css.

/* Header mobile */
@media only screen and (max-width: 767px) {
  .responsive #top #wrap_all #header {
    position: fixed;
  }
}
@media only screen and (max-width: 767px) {
.responsive #header_main .container {
    height: 50px !important;
}
.responsive .logo a {
    vertical-align: top !important;
}
.responsive .logo img {
    max-height: 50px !important;
}
}
@media only screen and (max-width: 767px) {
.responsive #top #wrap_all .main_menu {
    top: -15px;
    height: 45px;
    left: auto;
    right: 0;
    display: block;
    position: absolute;
}
}
@media only screen and (max-width: 500px) { 
.responsive #top #wrap_all #main {
        padding-top: 50px !important;
    }
}
/* Farbe Nachrichtenversand */
h3.avia-form-success{
 color:#99a502!important;	
}
/* Text mit Foto*/
.avia_textblock p img {
	  border-radius: 100%;
	  box-shadow: rgba(255, 255, 255, 0.6) 0px 0px 4px 4px;
}
.avia_textblock img.alignleft {
  margin-right: 40px;
margin-bottom: 20px;
}
@media only screen and (max-width: 767px) { 
#hello .avia_textblock img {
	width: 50%;
}
}
/* ScrollDown Pfeil */
#top .scroll-down-link {
	color: #a47654;
}
/* Blog Leerzeile entfernen */
.page-id-691 #after_section_2 {
  display: none; 
}
/* Blog Postdatum entfernen */
.page-id-691 .av-masonry .av-masonry-date {
  display: none; 
}
/* SocialMedia Icons*/
#top #wrap_all .av-social-link-instagram:hover a, #top #wrap_all .av-social-link-instagram a:focus {
    color: #fff;
    background-color: #c82161;
}
/* Icon Circle schneller*/
.avia_start_animation .avia-icon-circles-inner {
  transition: all 0.2s cubic-bezier(0.175,0.885,0.320,1.275);
}
/* Desktop Menu*/
@media only screen and (max-width: 2500px) {
  #top #header.av_header_transparency .av-main-nav > li.menu-item  {
      display: none!important;
  }
  #top #header.av_header_transparency .av-burger-menu-main {
      cursor: pointer;
      display: block!important;
  }
}
/* Header Button Desktop*/
@media only screen and (max-width: 450px) { 
.slideshow_inner_caption {
	top: -40px;
}
}
@media only screen and (min-width: 768px) { 
#top .caption_bottom .slideshow_caption .slideshow_inner_caption {
    position: relative;
    text-align: center;
}
#top .slideshow_inner_caption .avia-slideshow-button.avia-button {
    width: 500px;
    height: 56px;
    font-size: 22px;
}
@media only screen and (min-width: 768px) { 
.slideshow_inner_caption {
	top: -125px;
}
}
#top #wrap_all .slideshow_inner_caption .avia-slideshow-button.avia-button {
    background-color: #a37652;
    color: white;
    border-radius: 10px;
}
#top #wrap_all .slideshow_inner_caption .avia-slideshow-button.avia-button:hover {
    background-color: #c79a52;
    color: white;
    border-radius: 15px;
}
/* Header Button Mobile*/
@media only screen and (max-width: 767px) {
.home .avia-fullscreen-slider .caption_bottom .slideshow_caption {
    bottom: 250px;
}
.home .slideshow_inner_caption .avia-slideshow-button.avia-button {
    background-color: #a37652;
    color: white;
    border-radius: 10px;
}
}

I’m just not looking through in my css anymore. I’m afraid to make it worse
Can you please help me?

/* Header mobile */
@media only screen and (max-width: 767px) {
  .responsive #top #wrap_all #header {
    position: fixed;
  }
}
@media only screen and (max-width: 767px) {
.responsive #header_main .container {
    height: 50px !important;
}
.responsive .logo a {
    vertical-align: top !important;
}
.responsive .logo img {
    max-height: 50px !important;
}
}
@media only screen and (max-width: 767px) {
.responsive #top #wrap_all .main_menu {
    top: -15px;
    height: 45px;
    left: auto;
    right: 0;
    display: block;
    position: absolute;
}
}
@media only screen and (max-width: 500px) { 
.responsive #top #wrap_all #main {
        padding-top: 50px !important;
    }
}
/* Farbe Nachrichtenversand */
h3.avia-form-success{
 color:#99a502!important;	
}
/* Text mit Foto*/
.avia_textblock p img {
	  border-radius: 100%;
	  box-shadow: rgba(255, 255, 255, 0.6) 0px 0px 4px 4px;
}
.avia_textblock img.alignleft {
  margin-right: 40px;
margin-bottom: 20px;
}
@media only screen and (max-width: 767px) { 
#hello .avia_textblock img {
	width: 50%;
}
}
/* ScrollDown Pfeil */
#top .scroll-down-link {
	color: #a47654;
}
.html_header_transparency #top #main .caption_bottom .slideshow_caption {
    padding-top: 0;
}
}
@media only screen and (max-width: 767px) { 
	#top .caption_bottom .slideshow_caption .slideshow_inner_caption {
    position: relative;
    text-align: center;
}
#top .slideshow_inner_caption .avia-slideshow-button.avia-button {
	padding: 10px 6px;
	font-size: 10px;
}
}
.html_header_transparency #top #main .caption_bottom .slideshow_caption {
    padding-top: 0;
}
}
/* SlideShow Button Desktop*/
@media only screen and (min-width: 768px) { 
#top .caption_bottom .slideshow_caption .slideshow_inner_caption {
    position: relative;
    text-align: center;
}
#top .slideshow_inner_caption .avia-slideshow-button.avia-button {
    width: 500px;
    height: 56px;
    font-size: 22px;
}
@media only screen and (min-width: 768px) { 
.slideshow_inner_caption {
	top: -125px;
}
}
/* SlideShow Button Mobil */
@media only screen and (max-width: 767px) {
.home .avia-fullscreen-slider .caption_bottom .slideshow_caption {
    bottom: 250px;
}
.home .slideshow_inner_caption .avia-slideshow-button.avia-button {
    background-color: #a37652;
    color: white;
    border-radius: 10px;
}
}
/* SocialMedia Icons*/
#top #wrap_all .av-social-link-instagram:hover a, #top #wrap_all .av-social-link-instagram a:focus {
    color: #fff;
    background-color: #c82161;
}
.page-id-691 .av-masonry .av-masonry-date {
  display: none; 
}
.avia_start_animation .avia-icon-circles-inner {
  transition: all 0.2s cubic-bezier(0.175,0.885,0.320,1.275);
}
@media only screen and (max-width: 2500px) {
  #top #header.av_header_transparency .av-main-nav > li.menu-item  {
      display: none!important;
  }
  #top #header.av_header_transparency .av-burger-menu-main {
      cursor: pointer;
      display: block!important;
  }
}
.page-id-691 #after_section_2 {
  display: none; 
}

Thanks again Rikard!

Now I tried to change the color of the font myself:

h3.avia-form-success {
font-color: #eeeeee !important;
}

But that doesn’t work: the font color stays black.
What do I do wrong?
Thank you in advance for your reply.
Kind regards
Ronald