fix: make theme uploads more tolerant

[Xazin]

If there are missing keys in the light or dark theme file, we simply replace them with default values.

This is not a good way of handling errors, but it's better than failing indiscriminately without any proper feedback.

I've added a getMissingKeys to compare and see which values are missing, and written some quick unit tests for it. We should find a way to use it so we can let users know if their theme has missing values.

Relates: #5359

PR Checklist

• My code adheres to AppFlowy's Conventions
• I've listed at least one issue that this PR fixes in the description above.
• I've added a test(s) to validate changes in this PR, or this PR only contains semantic changes.
• All existing tests are passing.