Thanks @owen.campbell, I am not even close to being able to html or css efficiently enough but… I did find a similar thread (not sure how I didn’t find it before) which provides a work around. Thanks to @andersgs and @stucork for the css role solution.
The ColourChanger module contains some CSS - I got it by taking theme.css and picking every rule that had a %color substitution
The setThemeCss() function, which lives in a <script> tag under Native Libraries, is a two-line JS function that inserts a new <style> tag into your page, containing the new colour-changing CSS your Python has assembled.
Form1 shows you how to use this - it’s really simple! Just assemble a dict with the theme colours you want, and call set_theme()
Performing some thread necromancy here. For anyone who stumbles onto this while searching for a solution (like I did), I’ve created an all-Python solution to switch between the built-in color schemes. You can also add your own. Info: