The placeholder text for the QSearchField in the Collections tab is currently black, making it hard to see on operating systems using a dark Qt theme such as KDE Plasma. Perhaps a lighter grey colour could be used to make it stand out more and this colour would also not clash too much on light themes.
Also, the background of the search field is very close to that of the Collection background, making it blend in even more. Though having lighter placeholder text could potentially mitigate this, changing the colour of the QSearchField could make it more visible to users. For instance, the same colour as the search box above the playlist view.
I investigated potential ways to go about fixing this myself but I'm not that familiar with Qt or the QSearchField library, so I'll leave it open as a feature request.