Hi Lluís,
While making CSS customizations is indeed possible, it is not something we can help with much here—check out the “Product Support” section of this page for more information about his: http://theeventscalendar.com/terms
If you need to write custom CSS on your site, definitely check out a [free!] tool like Firebug if you use FireFox, or the Developer Tools for either Safari or Chrome. They have “Inspector” tools that let you zoom right over the element whose styles you would like to change.
As an example, you could use these tools to hover over mini calendar days that have events—and then you can just add some background-color styles to these elements to change their background to black, which appears to be the goal in your screenshot.
That CSS would look something like this—try pasting this into your theme’s style.css file:
.tribe-mini-calendar td.tribe-events-has-events {
background-color: #000 !important;
}
I hope this information helps!
George