CSS: Fix the top bar gradient to follow $PX_BRANDING_LARG
CSS: Fix the top bar gradient to follow $PX_BRANDING_LARG

This way even if we change that value, the gradient stays fit.

linuxfrorg/linuxfr.org,linuxfrorg/linuxfr.org,linuxfrorg/linuxfr.org,linuxfrorg/linuxfr.org