Css theme / block collapse image is missing after open/close a collapsed block

I have defined a new theme from a standard theme. I have make some changes in the fonts for grid and form.
When I apply theme to the applications there is a problem with the Block Caption.When I click on a collapsed block it is working but the image showing the collapsed state is missing.

With the standard theme there is no problem. The images below are the missing (not showing)!

tree_mais.png

tree_menos.png

I reselect the images from gallery in the theme editor and it is ok now.