Changing font size to percent in theme
This is the perfect place to use em instead of px for your font sizes. Change the font size in your body to font-size: 13px; or something, whatever size you want to be your standard size font.

Then, for all of the other elements, you can use em to adjust the size of the font based on percent. For example, font-size: 1.5 em; would be 1.5x larger than the standard size of 13px, and font-size: 0.5 em; would be 0.5x the size of the standard of 13px;