Fix issue where theme select does not appear when user's theme is deleted

[ZephireNZ]

Proposed change

Fixes #11701 where the theme selection is missing.

This is because the code is trying to check if the theme supports light/dark mode, but this won't work if the theme has been deleted.

This is a quick fix, and means the selector appears again. Perhaps there could be a better fix, where we reset the user's theme if it's one that has been deleted?

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

• This PR fixes or closes issue: fixes Theme selection field disappears, when selected theme is deleted #11701
• This PR is related to issue or discussion:
• Link to documentation pull request:

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

[ZephireNZ]

To recreate issue/test the fix:

1. add two different themes to HA
2. switch to a theme in your user settings
3. delete one of the theme from your config
4. restart HA, and attempt to change your theme