[UX] CustomCSS field for advance persistent styling

[arielj]

This PR aims to fix 2 problems:

• When customizing heroic, themes can change the style of the app but users may want to apply changes that are persisted between different themes (like hiding parts of the app, making different elements bigger, moving things around, etc). Currently, the only option to do that is to edit all themes to include the same style.
• The second problem to solve is to allow users the flexibility to customize things without the need of a full CSS theme and without the need for a specific configuration

For example, this feature can be used to support this feature request #4242 (which was also requested in Discord) by applying this custom CSS

(or display: none to fully remove those icons)

Then, any theme that's selected will still have this custom CSS style applied.

Since this can be dangerous (a user could body { display: none } and break everything), there's a warning with an indication on where to update the config files manually, and that's the reason why I put this in Advanced, it's not meant for inexperienced users.

---

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)