[UI] Add css variable for controller hints background

[wbrtm]

Adds a new css variable to allow the controller hint bar to be styled independently in themes.

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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)

[redromnon]

Looks better 👍

[flavioislima]

Looks good!