In QmitkSegmentationUtilitiesViewControls.ui a stylesheet is set (icon-size:32px 32px). Open file in QT Designer and filter properties for style to reproduce.
If you theme the application (e.g. dark) the set style property overwrites the given theme which results in standard coloring (everything is dark except of the SegmentationUtilities Plugin).
Please move the setting from ui file to default style qss file or so as all other plugins abandon setting the property that way.
;) Best Regards,