Due to the display scaling (125%), there are quite a few “imperfections” when it comes to icons (but also checkboxes).
As far as I know, (many of) these issues could be fixed by using icons and UI elements in SVG rather than PNG format, because the quality of vector graphics doesn't deteriorate when scaled up or down.
OSB Sudio is an open source project which is also based on Qt. The themes consist of QSS and SVG files.
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