Background
UberMenu loads its stylesheets last in an effort to override as many default theme menu styles as possible.
The Problem
Internet Explorer can only load 31 stylesheets per page – if a site tries to load 33 stylesheets, the last 2 will be ignored.
If your theme or another plugin is loading a large number of stylesheets, and the limit is reached before UberMenu’s stylesheets are loaded, UberMenu’s styles will not appear at all in Internet Explorer, because IE has ignored those stylesheets entirely.
Note: UberMenu loads two stylesheets by default, both of which can be disabled if you prefer to manually include those styles elsewhere.
The Solution
While this is a silly limitation of IE, if you have more than 31 stylesheets, you’re doing it wrong. That’s a lot of wasted HTTP requests when you could get all the data with far less communication with your server.
Either manually combine your stylesheets, disable plugins that are loading large numbers of stylesheets, or use a plugin to minify and concatenate your stylesheets into one mega sheet.
See also Asset Optimization