Although it's not possible to change the way text displays on the entire site, it's possible to customize the display of post content.

Choose any thread, scroll down to the bottom and click the Styler button. There are options for typeface, text size, text background and text colour. Click the green tick symbol to save your choices.

Note these choices will only change the text in posts and conversations, so you need to be on a page that displays them in order to see any difference.