101d8782f7
Since 115d809358 the scroll bars are darker on light theme. But this
makes the circular "handles", in Timeline for example, to not be
visible. This is because we start with the item color and then make
it darker when over a threshold. But we never make them lighter when
they are dark. This PR makes these darker or lighter depending on the
base color.
Pull Request: https://projects.blender.org/blender/blender/pulls/137618