Improve message when changing the theme

[tobiasdiez]

After changing the theme in the preferences (Preferences > Appearance), a message is displayed à la "Changed theme to: xyz", where "xyz" is "Base.css" or "Dark.css" depending on the new theme. These should be replaced by better names such as "Light theme" or "Dark theme".

Code implementation: Introduce enum for themes with the properties "display name" (localized), "name" (for storage in the preferences) and "css file name".

[jairus-crabb]

Hello, I am a new GitHub user and would like to tackle this issue!

[namanvats]

I am working on it.

[carlg0ransson]

Hey, I just created a pull request for this issue #5584

[tobiasdiez]

Thanks to @carlg0ransson this is now implemented in the latest development version. Could you please check the build from http://builds.jabref.org/master/. Thanks! Please remember to make a backup of your library before trying-out this version.