Thanks goes to GreatBull who found the solution.
This patch also force the icons to be displayed in the buttons for the Linux system (already worked fine on Windows)
A new checkbox has been added in the preference window to switch to a "slim" version of the selected theme, for low resolution screens
Curves now uses the theme colors too (was hard to see before with dark themes)
Themes has been named with this convention (of mine, yes) :
[Background ligthness]-[background color]-[Selected items color].gtkrc
Only themes with .gtkrc suffix are now displayed in the combo box
