Fix #1907: menus don't have a dark theme anymore

[Abykin]

This fix fixes Thimble issue https://github.com/mozilla/thimble.mozilla.org/issues/1907

This line was erased in one of the previous PRs : $("body").toggleClass("dark", theme.dark);
Putting it back resolved the issue. I did some local testing to make sure nothing broke, everything seemed okay.

[Pomax]

this fix seems to assume a binary theme choice in that if there is any theme at all, it must be the dark theme. That will break the moment we introduce a third theme, or custom theming, so it would be better to check which theme should actually be active. Maybe @flukeout can help point in the right direction here

[flukeout]

We intentionally moved away from the using classes to set the theme, and opted for a data-theme attribute on the body instead. We may have more than two themes in the future, so the dark / not-dark binary choice doesn't make sense for the long term. Maybe we can add an override to the quickEdit CSS so that it respects the data-theme=dark attribute, and not only the class name. Will get back to you shortly on this.

Cheers!

[flukeout]

After thinking a bit more and chatting with @Pomax - this will be the way we handle it for now. Brackets basically has a hard-coded notion of having a 'dark theme' and the .dark class is peppered throughout the codebase. My previous attempt to move the theme to an attribute mostly worked, but didn't anticipate a lot of cases where .dark still mattered.