Various fixes to the dark theme

[calixtus]

Some visual bugs were still in the dark theme present. This is a quick fix:

• Many icons where dark black. They use the standard text color now.
• Wrong highlight color of the mark symbol in menus is now highlight text color.
• The thin white line between the menubar and the toolbar is gone. ( fixes White line on screens scaled != 100% #5753 )
• The background on starting up JabRef was white until a library was loaded.
• The unselected checkbox border was black, only while the unselected radio-buttons were grey. Now they are all grey in various shades. (less then 50)
• The group color indicator was unreadable, as the grey border was too ... grey. It's more lightweight now.
• The CodeArea in the in the entry source editor and in the preview preferences had the wrong background color, caret color, selection background color, line number colors and some highlights were unreadable. They use the common color scheme now. ( refs #5522-comment )
• The citations tab was bright white, had some windows specific fonts encoded and some obscure font size settings. It uses the common color scheme now and the font settings are a bit equalized.
• The dark tab header bar in the preview preferences was too heavy. It uses the common background color now.
• The related articles tab in the entry editor presents the gdpr warning some smaller font size, it wraps according to the window width, shows a heading and has a fancy yellow border around it.
• Fixed a typo ("Mr DLib." instead of "Mr. DLib")
• The TexParserResult dialog is now presented in a SplitPane.

Except the Mr. DLib tab, the light theme should not be affected.

Some screenshots before:

And after:

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for bigger UI changes)
• Checked documentation: Is the information available and up to date? If not: Issue created at https://github.com/JabRef/user-documentation/issues.

[Siedlerchr]

Thanks for the good work! Loosk good to me! I merge now.
It's just a bit confusing that github actions are not running on upstream but on your fork when you create a PR.

[calixtus]

Maybe GitHub likes me more than you. 😁
No, in fact it's annoying to me too, because I get an email every half an hour, that a build could not be uploaded to builds.jabref.org .

[Siedlerchr]

@calixtus That's the first thing I disabled: https://github.com/settings/notifications