[Feature Request] Add the option to edit custom themes in settings

[AdilDevStuff]

I know that you can edit the godots.cfg file to edit custom themes but it would be really helpful if the default/common entries can be edited from the settings.
Sometimes the cfg file behaves weirdly, resulting in having to edit the cfg file multiple times which is annoying. I tried to do this myself but I m too noob to figure out the code lol

Will really appreciate if this can be added!