Custom extended themes

[Bonsai8863]

Proposed changes

• Added new Theme objects in backend, along with relevant resolvers and such
• Modified Settings page to allow admins to create, modify, export, and import custom themes
• Modified Settings page to allow admins to select default platform theme from a dynamic list
• Modified User profile page to select their preferred theme from a dynamic list
• Modified frontend theming to use dynamic themes instead of dark/light themes

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case (coverage and e2e)
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

Related to the discussion in #8239, we decided that a better solution would be to allow admins to customize the themes of the platform with greater granularity. This should eventually expand to more fields, such as text color.