Add dark UI theme and custom theme support

[kaygdev]

Enable user CSS and dark mode for the UI from the settings dialog. Fix some graphics issue with dark mode described in #466.

For the dark mode is used the dark orange stylesheet from http://discourse.techart.online/t/release-qt-dark-orange-stylesheet/2287 with small modifications.

[JARVIS-AI]

Did you test it ? (Run it with your changes)

[trollixx]

Bundling unmaintained styles is not a good idea. Any operating system or Qt update will potentially break it. Also it would require extensive testing on various systems with different settings.

If something in Zeal breaks custom styles, that needs to be addressed, and I'd be happy to accept such changes, but otherwise styles should not be part of the project.

There's a ticket #811 about supporting dark window style.

[trollixx]

This needs to be revised to only contain changes required for custom styles to work, but not specific to any style.

[0xboz]

I was trying to add the setup I used (Debian 10), but it was closed by the bot. I guess I will just leave it here in case someone comes across later.

sudo apt install qt5-style-plugins
echo "export QT_QPA_PLATFORMTHEME=gtk2" >> ~/.profile

Log out and log back in. Here is the result (on Debian 10 + Gnome + Flat Remix GTK )