How to make app responsive to Light and Dark modes?

[Marion]

With the advent of light and dark modes, how do I make my app respond to that? Specifically, how do I make the icons change color so they are visible?

As an example, I've noticed that LibreOffice has several black icons on their toolbar that are almost impossible to see when I have my PopOS computer in dark mode. I'd like to have the icons change color (CSS) to a lighter color when the environment is running in dark mode.

[KodeZwerg]

For Windows you can see my (Delphi) try to do such here.