Change font size on tabpages ('guitablabel') on vim?

I'm using gVim on a linux machine, and would like to have smaller fonts on my tabpages (guitablabel). How do I change t开发者_StackOverflow中文版he font sizeonly on tabpages(tabs)?

Now, my config is

set guitablabel=%t

A quick test in Ubuntu shows me that this is determined by your theme preferences, not by gvim.

To clarify, I mean GTK theme (or whatever Ubuntu's appearance preferences change), not vim colorscheme or anything like that.