Site Theme Overhaul

[majora2007]

Since I first built Themes into Kavita, it has always been my plan to make a viewer to make it easier on users to get and use new themes within Kavita.

In addition to making this change, sticking with the theme of keeping things simple, the existing mechanisms for adding custom themes (outside of Kavita's repo) will be overhauled to reduce having to interface with the underlying filesystem.

• Ability to browse themes repo for themes to download and apply
• Ability to sync against themes repo and get the latest theme (assuming theme compatibility matrix matches installed version)
• Ability to delete a theme from UI
• Rework theme deletion to move the file to temp to give the admin time to recover in case of mistake
• Update backup to ensure themes are backed up
• Give the user ability to select a css file on disk and upload it manually, instead of using the browser.
• Rework UI so themes have some sort of cover image or primary colors displayed on the card