Easy way: look for "Author mode / User mode" switch, which should be located somewhere around your user interface. If it's not there, you can find it in the main menu bar, under View / Style. There you can pick various pre-made styles what might be more readable for you.
Then there's a more elaborate way, if you know how to work with CSS files: Right click the current page and pick "Edit user preferences" in the menu. There, under "Display", you can pick another pre-made stylesheet. That way, you can change the style of whole site to completely different design.