Event Calendar 'Theme' Style Sheet Add-In:

Simply copy the contents of 'theme_css_definitions.txt',
into EVERY 'style.css' in EVERY theme directory that you
are using. Doesn't matter where in the 'style.css' file
you paste it; beginning, middle or end. Depending on the
Theme in use, you just need to edit the element colors in
this 'style.css' addition to your Themes css file.
Then go to the 'calendar settings' and enable 'Use custom css'.

Thats it ! Enjoy!