gianpiero,
Sorry that you are seeing this issue. Thank you for the screenshots. It looks like the problem is related to the OS themes. I could not replicate the reported problems with Darkest Dark plugin in Windows OS.
From Linux Mint System Settings, go to Themes, click on the current Controls theme, select a different theme and check if the problems persist.
If you are seeing the problem with only specific Controls theme, then please list them to help us fix it for those themes.
Apologies for inconvenience caused. Thank you for reporting it.
–Swapna
MyEclipse Support