See DNN – Custom CSS Stylesheet for FCK Editor for how to set up a customized style sheet. To avoid having to maintain redundant style sheets between your skin.css and your FCK Editor style sheet, place a reference to your FCK Editor style sheets at the top of the portal.css file stored at the root of your portal.

@import url("fckeditor.css");

Write all the styles that are relevant to your html/text module areas into the FCK Editor style sheet not your skin.css. Your customized stylesheet will be available to FCK Editor as well as your document and you’ll only have to maintain your styles in one file.