i did not try this before, but most of the styling could be edited in the Sass files, like fonts, colors etc.
Are you familiair with working with Sass?
If not, there is a great learning path available in the Mendix Academy.
The simplest way to change your font color/ style/ size can be done in the Mendix Studio. Choose the brush on the left navigation bar. The change will be applied to the whole application.
Otherwise, you can edit your own custom style in theme > web > custom-variables.scss