By adding the folowing code I succesfully made teh white squares transparant:
Render class for menustrip:
public override Color ImageMarginGradientBegin
{
get { return Color.Transparent; }
}
public override Color ImageMarginGradientMiddle
{
get { return Color.Transparent; }
}
public override Color ImageMarginGradientEnd
{
get { return Color.Transparent; }
}
Altho the text isn't aligned to the left. But that's not that much of a problem. Credit to Franck aswell!