fix: allow partial overrides of CSS classes

[spike-rabbit]

What kind of change does this PR introduce? (check one with "x")

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

Although the type says diffrently one always needed to set every CSS class.

What is the new behavior?

Now one can only override partial CSS classes and the other defaults will be kept.

Does this PR introduce a breaking change? (check one with "x")

• Yes
• No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

This is part of a larger refactoring to enable strictNullChecks

[fh1ch]

@spike-rabbit really nice work here, thanks a bunch 💯

The changes are all clean and reasonable, no findings from my end.

LGTM 👍