In you custom.scss file add
.mx-groupbox-header {
background: #333; //the backgroundcolor
border: 1px solid #ddd; //the bordercolor
color: #FFAA00; //the font color
}
then generate you css using either Calypso or commandline sass in the theme/style directory:
sass --watch sass:css
and restart your project.
Absolutly, this is possible using CSS. More information on how to change the styling in a Mendix app can be found here https://docs.mendix.com/howto/front-end/style-with-gulp-and-sass#1-introduction, and https://docs.mendix.com/howto/front-end/calypso, and https://gettingstarted.mendixcloud.com/link/path/28