Thanks for testing S.Ali.
At the moment I believe that the background should be the hover colour and hence as such ensuring that all places follow that. If in future it needs to change, then at least it will be a consistent change. I've patched things in V2.9.0.4 to be better for alternate colours.
With the 'Participants', that's already in M2.8: https://github.com/gjb2048/moodle-theme_essential/blob/MOODLE_28/less/essential/fixes.less#L14-L17 and as such is not a change from the M2.8 version. So I'm not going to change it in this circumstance as the style is reactive to screen size and being inline is an improvement over Clean and saves space.
Cheers,
Gareth