CSS increase font size

CSS increase font size

I'm a no expert in CSS, but have this task to increase font size on the
site. The site uses a downloaded CSS-theme. As I see there is a single
file main.css which contains definitions of fonts. In other css-files
font-size is set using percentages.
However, there are 102 matches for the word font-size in main.css itself,
because it sets sizes for all possible html elements and their
combinations, like
body { font-size: 13px; }
h1 { font-size: 32px; }
h1.smaller { font-size: 31px; }
h2 { font-size: 26px; }
and so on.
I am thinking to write a script that would extract values of font-size $1
and replace them with $1+1.
In a while, probably there is a more elegant solution? Maybe I can
redefine font sizes some way using CSS itself?