Transparency of windows that are NOT in focus is a "feature" of Peppermint 4 .. you can disable it if you prefer by going to
menu > Preferences > Window Manager Tweaks > Compositor (tab)
then adjusting the "Opacity" slideer for "Inactive Windows"
As for Chromium, either maximise the window .. should remember that after closing/reopening Chromium
or
Position the Chrome Window to top left, then drag the bottom right corner of the window till it fills the screen .. again, closing reopening should cause it to "remember" the window size.
If neither of these work, we can change the command for opening chromium to include the
--start-maximized
option, which will force Chromium to always start maximised .. if you need help with this, just ask
