The default htmlArea RTE in TYPO3’s backend renders the textarea content in a pretty ugly fashion. It looks like in the HTML 1.0 ages. But you can change that using just a few lines of code and a css file. How to accomplish this you might wonder. Its easy.
Go edit your root page (or some other page if you want to do this only on a page) and in the Options tab there is a TSconfig field, now just paste the following code there:
contentCSS = fileadmin/templates/styles/rte.css
Now you just be sure to have a rte.css file in the specified location (fileadmin/templates/styles) and you can put there your css definitions for the RTE text. This way you can match the RTE’s look and feel with your site’s design.
Before using CSS for RTE
After using CSS for RTE