This is likely due to
custommenu.css being loaded after your custom theme. One way to combat this is, instead of using page.xml, to put the declaration in
local.xml of your theme's layout folder. This will be loaded last and therefore your css overrides will come last.
A better way:
Another way to handle this is to create a custom layout file and reference it from a local module's
In the module's
app/etc/modules/YourCompany_YourModule.xml declaration you will set its' dependency to whichever module is adding
Your new file
custom.xml can have the css declaration. This may seem a bit extreme but in my opinion is more correct, avoids local.xml declarations, and properly sets the dependency.
Another, nother, way:
This is the brute-force-getterdone method. First, check where
custommenu.css is coming from. It's likely from something similar to
skin/frontend/base/default/css/custommenu.css - if so, your local theme can provide an override.
Copy the custommenu.css file to
skin/frontend/yourpackage/yourtheme/css/custommen.css and make your local edits there. Ba-blam: now you've overridden the file without disturbing the original file itself.