leoSettings.leo contains a "Gutter Fonts" node, with four settings, but you probably already know that.
One of those settings is @string gutter-font-size = @small-font-size. Did you change this setting?
Searching through Leo's core sources, I see that Leo only uses the @bool use-gutter setting.
I suspect that your theme's stylesheet doesn't support gutter-font-size. My personal style sheet contained:
QFrame#gutter {
background: gray;
}
Changing this to the following worked for me:
QFrame#gutter {
font-size: 20pt;
background: gray;
}
HTH.
Edward