Happypanda lacks a "default" theme

[nonamethanks]

While testing different qt themes I realized that happypanda does not really have a way to handle dark themes. These often make some lines of the program ugly, very hard to read, or depending on the theme even just impossible to discern from the background.

Several qt-based programs "fix" this by offering a default color scheme that the user can select as a replacement for the system default (a perfect example of this being Calibre).

Happypanda should also probably offer a way for the user to fall back to something that's guaranteed to work if the theme they're using is incompatible with the program, by making the default theme to be completely independent of the system theme, or at least a way to pick a different theme only for this application without having to switch to another color palette for the entire system (kinda like the --style=theme_name flag that qbittorrent offers).