Revert "gtk3: CSS's font-size needs an unit, assuming pixel"

This reverts commit 293be58bfb7455d8b1ca91150e9f4a912e6ab124.

It seems that older versions of Gtk don't support setting an unit here,
which makes Sonata crashes.
I have no idea from which version this support has been added, nor when
it started to get deprecated, so I'm just going to forget this warning
for now.

Fix #40