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.
Best Answer
I found my answer when discovering that (without knowing it) I had installed Gtk+ 2 Theme Change, I selected the same theme there as the one in LXAppearance.
How I exactly got the program I do not recall. Maybe it was installed along with a theme that had an installer or a series of instructions that I followed without taking notice.
The issue looks similar to a Qt-related problem which received a similar solution (see the askubuntu question).
The rule should be: The theme selected in LXAppearance and that selected in 'Gtk+ 2 Theme Change' has to be the same. (The quality of the way folders and fonts are displayed seems to have improved too.)
What happens is that I can set correctly a dark-menu theme just through LXAppearance and I can do the same thing for the light-menu themes in Gtk+ 2 Theme Change. But sometimes it happens that setting a darked menu theme just in the later would make the fonts of the menus as dark as the background. So, the rule applies both ways.
The program is available in Software Sources (search Synaptic for 'Gtk+ 2 Theme Change')
On Softpedia here.
And the website of the provider.
It is in Software Center