Revert font setting in custom class

[vkbo]

Summary:

This PR reverts a change from #1940 that uses a font from theme when setting the default font in the custom colour label class. This caused some strange behaviour likely due to a loop somewhere when loading the Preferences dialog, which uses it multiple times.

Related Issue(s):

Reviewer's Checklist:

• The header of all files contain a reference to the repository license
• The overall test coverage is increased or remains the same as before
• All tests are passing
• All flake8 checks are passing and the style guide is followed
• Documentation (as docstrings) is complete and understandable
• Only files that have been actively changed are committed