Hi. I found the link to that screenshot: http://www.heilhauslueneburg.de/termine/monat/
This CSS code should change both the background color from #666 (dark gray) and the text color from #e88206 (orange). Obviously, you could choose to change them to whatever you want or leave one unchanged.
body.post-type-archive-tribe_events #tribe-events-content.tribe-events-month .tribe-events-tooltip h4 {
color: white;
background-color: black;
}
You can add custom CSS code via your child theme’s style.css file or via a plugin like Simple Custom CSS or, my favorite, Jetpack. Jetpack supports regular CSS and the LESS and Sass pre-processors.
If you’re not a CSS expert, Jetpack’s Custom Design reference may prove helpful.
Please let me know how it goes for you.