With the high contrast setting "contrast white", the highlight color is purple an the Text white:
{F21093918}
The buttons in Kleo do not honor this, e.g.:
{F21093941}
{F21093899}
{F21093952}
The button on the left has a lighter purple because it's background color is fading in and out.
I suspect it is no good idea to have the pre-selected buttons change background color this way as the text color needs to change in this case, too.
The pre-selected Windows explorer buttons do not do this, they are simply purple with white text.
This is similar in Windows 11 with the high contrast design "Desert".