In InDesign plugin development, how do you set the font size and color of controls?

Hmm, haven't tried that in years.

You can/could create derived boss classes that aggregate IID_IUIFONTSPEC, kUIFontSpecImpl and IID_IUICOLORSPEC, kUIColorSpecImpl. Also extend their types following the examples of widgets.fh so you could preset values in fr resources. Sometimes the IControlView implementation picks them up.

Looking across my sources my widgets that support those interfaces also implement an own IControlView, could be that this feature was dropped when the UI was "modernized" to maximum boring/neutral.  E.g. ScriptUI also lost the means to render specified colors years ago (when darkUI came along?).

Also note that some of the UI color constants differ from used values, and most font constants produce the normal style even when their name still mentions "bold" etc., variation is mostly in size.

The remnants of traditional widget UI are now emulated on top of another library "Drover" rather than wired to the OS. Only few of the Drover API is exposed in SDK – some DV wrapper methods, and it uses a different color theme.