For most tasks, the normal Cascade editor can be used exclusively. However, Cascade users can also edit HTML code directly by selecting the HTML button. When selected the HTML code window opens (view the image below). Once the HTML code is edited in the "HTML Source Editor" window, the Update button closes that window and updates the Cascade page.
All UM Cascade sites use several generally defined CSS files, but one is placed in every site also. People who are familiar with CSS can edit the site's CSS file. Because the local site's css file is referenced after the global css files, it will overwrite the global styles.
It is strongly recommend that beginners don't experiment with this file. To learn more about cascading style sheets, it is best to practice on files created on a local computer and displayed only on that desktop - at least at first. To learn more the W3Schools site is older but a good introduction.