Hello Emmanuel,
Good question. Depending on which part of the events calendar you want to change colours for, you can view our themer’s guide to customizing the calendar here: https://theeventscalendar.com/support/documentation/events-calendar-themers-guide/ (you want to scroll down and pay attention to the heading that says “CSS”).
It will also help to install Firebug on Firefox or use Chrome to be able to see the CSS elements of the page you want to modify.
Not knowing exactly which part of the calendar you want to modify (i.e., the whole page, the list, the grid, particular events, etc.), here is a sample of what you could try adding to your theme’s style.css (preferably your child theme’s style.css, if it has one):
#tribe-events-content {
margin-bottom: 48px;
padding: 1px 20px;
position: relative;
background: #ENTER-COLOUR-HEX-CODE-HERE;
}
Let me know if this works out for you!