Did you use a Mendix 2.4 or 2.5 theme-package?
As it said in the Moving from 2.4 to 2.5 guide:
Styling applications using CSS has been completely overhauled in version 2.5 and for this reason themes created for 2.4 have to be updated for version 2.5.
Unfortunately you have to re-create your theme. To do this please take a look at the tutorials. These can be found here. On the bright side, it's now easier to create your own theme and the client is a lot faster with the new HTML structure.
If you have made a new 2.5 theme and you want add this theme to the theme-switcher, it's possible to add the name (index-themename.html) to themes element. At the end of this week we will release a how-to for this solution.
<div dojoType="mendix.widget.ThemeSwitch" themes="Custom;Dutch;Dark;Default;Nostalgia" label="Pick your theme:"></div>
you should put your theme.zip in [project map]/theme It wil work both with a packed zip file and an unpacked zip file
I've placed my theme package in the [project map]/theme folder and nothing has been changed. After deployment 4 themes are visible on the right upper side of my browser. I can switch between the themes but I can't see my own package. How can I set my theme in Mendix?