This is very exciting! Congrats on the plugin. How will it interact with other CSS? Will userstyles.css etc also load?
I was also wondering about how this could interact with the CSS that other plugins use, such as --joplin-background-color etc that are seem to be defined within a plugin iframe (and are therefore not able to be affected by custom CSS at the moment). It would be really nice if you could make a 1-1 mapping with your colors so that plugins "automatically" (if built to spec) would use your theme colors. Probably out of scope for this!