I can highly recommend taking the course on:
It will teach you all the basics of HTML and CSS
In addition to the other posts, here's a webinar on styling basics with Mendix that you can watch:
Hi Brent and welcome!
My 2cents: Get acquainted with the HTML basics first, so that you get to understand what CSS will achieve in relationship to it. Meanwhile you can learn Mendix as it's perfectly doable without the HTML/CSS knowledge.
I remember in them old days that O'Reilly had some good books for HTML and CSS.