Theming: override input labels default so they use the MUI secondary color

[camillevilla]

Override text input labels so they use our secondary color instead of primary? See the light grey color used for this picker's label. Might need to check in with @jvine and @ggeisler about this.

[ggeisler]

I don't think we'll want to use the secondary color for the label, just a darker gray. I was hoping the on text colors would automagically adjust correctly to our color scheme but it looks like that is not the case.

I'll take this into account in the current work I'm doing on extending the color palette and we can revisit this when that work is ready.

[ggeisler]

I think this issue has been superseded by other theming related issues, and the specific example above is no longer relevant, so closing this.