Hi there, thanks for your feedback, I'm glad you like this light theme :)

Regarding the dark context menu in Proton: I'm trying to figure out how to change it to light mode and haven't found a way yet, but in the meantime, there's an option to turn off the Proton menu in about:config and search for browser.proton.contextmenus.enabled and change to false which will revert the context menu to pre-Proton and will change it to non-dark.

I think Proton context menu is dark by default (at least on my laptop it is the case, and it remained dark even when I switched to other themes).

I've found this workaround to force Proton context menu to change to light or dark mode, but it requires customising userChrome and about:config in your Firefox (independent from the theme):
https://github.com/MrOtherGuy/firefox-csshacks/blob/master/chrome/menupopup_forced_color_schemes.css

So far I haven't found a way to build this into the theme yet.
I'll keep looking for a way to change it, but if you do find the solution to this please let me know.

Firefox theme relies on specifying properties inside a manifest.json file, as per documentation here: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json/theme
I don't see any properties to override Proton context menu theme described here unfortunately.