Increasing the sheet name tab size for libreoffice calc in ubuntu 12.04

In libreoffice calc, the font size (or horizontal height) for the sheet name tab is related to the scrollbar slider width.

Open "/usr/share/thmemes/[a theme used, e.g. Ambiance]/gtk-2.0/gtkrc"

find "GtkScrollbar::slider-width = xx", where xx is a number.

then, change the number xx to a larger value.

Logout and login, or, (this is better) change theme to other one and then return to current theme using ubuntu tweak.