As we can see from the attachment systemtray.png, the background of the systemtrayicon is black. It's background color supposed to be as same as the panel's.
|For Gerrit Dashboard: QTBUG-35832|
|88839,8||Support transparency in qt5 system tray icons, regardless of tray visual||dev||qt/qtbase||Status: MERGED||+2||0|
|101557,2||xcb: Fix transparent tray backgrounds with 24bpp tray visuals||dev||qt/qtbase||Status: MERGED||+2||0|