You need to go to X > Theme Options > Typography. Also, I suggest that you enable and use the Font Manager.
Having said that, there are elements that have their own font size and color settings, such as the Text Element. If you do not want to set the font size and stuff each time you want to use the Text Element, for example, you need to use the Template Manager feature. Do all your settings and fine tuning once, and save the Text Element as a template. Then set it as the default of the Text Element. All the information is described in details in the article below:
Also, please read the article below for the Color Manager feature: