Buttons contrast isn't good when changing the Color Theme from the default one

[ane-gabriela]

The contrast between the background and the text color of the File Manger buttons is really poor, which makes the text almost unreadable, when changing the default Color Theme.

Eg. selecting Superhero (Administration > Look & Feel > Themes) leads to:

Eg. selecting Yeti:

Environment: Windows 10, XWiki 11.7 with MySQL 5.7 and Chrome 77

FYI - More info on this from Anca as we found this issue also on client testing "The cause of this issue is a choice of the pair of colors in the FileManager css with a pair that does not have the contrast ensured, Buttons are the color of default buttons (because bootstrap) while the text is colorTheme.secondaryColor . I think this property does not even exist anymore in the Flamingo color themes (it's a legacy variable from colibri), which are initialized with default values, so we cannot fix it from color theme config."