The built-in themes are compressed with the program files, so you can't edit them on disk. However, for purposes of creating your own theme, or possibly for use in Firefox Color, you can access their files on the source code site:

https://searchfox.org/mozilla-release/source/browser/themes/addons/dark/manifest.json

https://extensionworkshop.com/documentation/themes/static-themes/

See also this resource link for the built-in themes:

• resource://builtin-themes/

Thanks for your replies.

I tried to create a copy of the default dark theme (for personal use, to customize it later), but in order to install a theme on Firefox standard, I need to sign it. I cannot sign it due to the use of theme_experiment in manifest.json, because according to the docs: themes using experimental features cannot be signed.

I know that I can switch to Firefox DE and disable the check by setting xpinstall.signatures.required flag to false, but I'd prefer staying on standard.

Is there any way to easily create the default dark theme copy? or the only way is the hacky way: https://stackoverflow.com/a/42403531/6771167

How essential is the theme_experiment section? Or in other words, if you don't have that section, what things do not have the proper colors?

It seems that everything works just fine without it (maybe it'll break in the future, I'm not sure, but these solve the issue for now. Hope, I won't get confused for hours about why something is not working as intended in a few months :D ).