Since the merge of the new Graphical User Interface into the code trunk, the following themes are no longer supported/working in the current development build of ScummVM. Because of the massive overhaul on the theme description files, they will not work again unless their authors decide to port them to the new format.
See here for theme specifications and here for the TODO page.
The themes have their ini files in the zip archives, so they can just be placed in your theme folder. Note that the files may not work after renaming, so keep the filename. Also note that some of these themes could be outdated, so please wait for the author to update them, if they aren't working.
Modern Dark Theme
Modern Ice Theme
Modern Blue Theme