idk if this is what youre talking about, but a feature that CSS already has is media query selectors, which you can use to have different styles depending on the screen size! this could let you have different styles or show/hide certain HTML elements on mobile screens :D
so if you wanted a custom theme for mobile (which would be like 600 pixels wide) you could do this:
@media only screen and (max-width: 600px) {
/* put mobile-only css in here */
}
@media only screen and not (max-width: 600px) {
/* put desktop-only css in here */
}
here's a w3schools page about them: https://www.w3schools.com/cssref/css3_pr_mediaquery.php
and if you wanted a preview, you could press F12 on your browser to show developer tools, and then press this button in the top left:

(this is what it looks like on chrome, firefox has something similar in the top right)
then you can change the dimensions and emulate different devices at the top of the page
if you have any questions let me know!! (i'm brand new to spacehey but i've been using HTML and CSS for a long time) :)