Hi Jürgen,
If the preview form shows the change even after changing, then it’s defintely a cache related issue only.
If not, then it might be a javascript issue, browser plugin issue, or something related to those perhaps. In this case, can you please try it in a different browser? Preferably Chrome or Firefox. I don’t recommend using IE for back-end for any plugin, often times it causes issues.
Alternatively, I would recommend this custom CSS code:
[html].wpdreams_asp_sc, .probox {
background: #fefbd8 !important;
}[/html]
This targets most top level element backgrounds, including the search form.