Allow using custom CSS files to theme Heroic

[arielj]

This PR add the feature of using custom CSS files as themes in Heroic.

I'll add some details about naming conventions and limitations here https://github.com/Heroic-Games-Launcher/HeroicGamesLauncher/wiki/Custom-Themes in the next few days

How it works:

• users can select a path to store their .css files
• once set, the .css files will be listed in the theme selector
• when a theme is selected, if it's not one of the defaults, a <style> tag is injected at the top of the body element with the content of the .css file

custom-themes.mp4

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Use the following Checklist if you have changed something on the Backend or Frontend:

• Tested the feature and it's working on a current and clean install.
• Tested the main App features and they are still working on a current and clean install. (Login, Install, Play, Uninstall, Move games, etc.)
• Created / Updated Tests (If necessary)
• Created / Updated documentation (If necessary)

[flavioislima]

That is awesome, a really simple approach and works really well.
I think now we will need a website to share user themes, ahahaha.