I started using Lubuntu (12.04) and I like the customizations that I was able to make, especially fitting icons and themes ('Widgets' - as 'Customize Look and Feel'/LXAppearance calls them.)
But for some reason about a half of the themes make my menus display a pale font on a whitish background - and look unclear. So, I am stuck with customizations involving themes that are darker or that otherwise don't lead to this.
I am aware only of some but important menus that are affected by this, like the Main menus, Right-click context menus of File browsers and the Firefox right-click and drop-down menus. The right click menus of the tray items are not affected, nor the password area, nor other important parts in File browsers or applications, as other people experienced and reported in askubuntu.
I think the problem is more with the colour of the fonts than with the background and that some small tweak might t solve this. I think lubuntu is not concerned by the issue of compatibility between themes and gtk3.