I have one of many programs that change Windows to dark/night mode.

Trouble is, the custom color for the clocks on the secondary taskbars is black (in light mode they're white). The taskbars themselves become black/dark when Windows changes to dark mode.

As a result, the clocks effectively vanish.

How feasible would it be to either have light/dark settings for these clocks' colors, or automatically invert the color if DF sees the OS visual mode change?

If you open the Advanced Settings and scroll down to the Taskbar Clock Colour you can change it with RGB to any colour you want.
This FAQ tells you a little.
https://www.displayfusion.com/AdvancedSettings/#taskbar_clocktextcolour