Your question is not OJS related, is more a CSS question… and your OJS version is old, but let’s see what we can do.
if you like to change the font size, you need to change “font-size” property in your CSS.
In the styles you are publishing you modify h1 sytles (in #header) and #leftSidebar but looking to your site I think you want to change the font size for all your content that now is 0.7em.
Try adding this to the end of your CSS and play with the font-size value to see if this is what you want: