How to adjust colors of title text and header in default widgets?

First of all, greetings to the board, I am new here and hope my question is not a stupid easy to make mistake.

As you can see from the picture below (Latest Topics Widget), the color of the bar (two blue tones) and the color of the title font make the default widget title very hard to read, which led to me disabling the widgets for now (all widgets show the same color pattern like this).



My question is now, how to adjust these colors? Is there an 'easy' way?

I was guessing that the header is some image, but didn't find it in the advanced image interface. Also my search in the advanced color interface didn't come up with anything, as I cannot pinpoint what I'd need to change. The CSS didn't help either, as I haven't been able to find anything that catches my eye with regards to those default widgets. Furthermore, I tried a search on forum here, but had no luck so far.

Thank you in advance for any hints and help, highly appreciated.