background image overlay – on mobile only

hi there,
we’re using a color section with a background image. within the color section is a heading, a text block & a button. i’d like to change the opacity of the background image only (not the whole div).
e.e. opacity: 0.8;
i’d like this only on mobile – not desktop/tablet.
is there any way to achieve this, obviously checking the enable overlay box affects every device.


i’m using a child theme, so happy to make mods to template files if css won’t do it.
many thanks,
Jason