When using the builtin Windows 9x theme with KDE and a dark color scheme, it seems to be that 50%+ of elements in the KDE system settings don't render any sort of keyboard focus at all, requiring to press tab followed by space or arrow keys to mess around until something reacts and you figure out what is focused. Some other elements seem to render the keyboard focus using a wrong color, using black on dark gray rather than white. Only the search box seems to work as intended

Strangely enough, most popup dialogs seem to have a perfectly working white on dark gray and easy to see keyboard focus on all the buttons, so this seems to be some KDE system settings problem. However, e.g. Spectacle is also affected.

When changing the theme all issues go away. It was suggested to me this is a "MS Windows 9x" theme problem.

https://bugs.kde.org/attachment.cgi?id=177519 Screenshot with annotations illustrating the affected areas where keyboard focus seems to misbehave

https://bugs.kde.org/attachment.cgi?id=177520 Screenshot showing that in some popup dialogs, the keyboard focus seems to work as intended

Original upstream KDE bug: https://bugs.kde.org/show_bug.cgi?id=498869