There is a bug report here (Bug 213193). Unfortunately it doesn't look like you can expect this to be fixed any time soon...
Section#setTitleBarBackground
is working now - sets the top color of the section background gradient like in RCP.
Section#setTitleBarGradientBackground
sets a color (Section#COLOR_GBG
) that is never used in Section class??!!
Section#setTitleBarBorderColor
- has no effect.