I must be missing something. Either my comprehension level has suddenly hit rock bottom, or the instructions are a bit to vague. I am trying to use the Nice_Menus module (6.x-2.1) with Drupal 6.19 on a flash drive loaded Uniform Server 5.6.1-Nano i use for development. When it comes to customizing the menus, the documentation suggest copying and renaming "nice_menus_default.css" then using that one to customize the css. To do that you point to that file on theme configuration page "Administer -> Themes -> Configure -> Global settings -> "Path to custom nice menus CSS file". After doing that the menus broke, i.e. children appeared behind parents, no borders, margins, padding etc. I thought it was my theme, but the problem still occurred with Garland theme. I had not changed anything in "nice_menus_default.css" other than renaming it. I can change the nice_menus_default.css and get results for css changes, but the custom css appears to have no effect. I have looked through the issue cues and have not found this problem. I have searched the internet, but most of it says the same thing as the documentation. What an i missing? I had put the custom css file in the same location as the nice menus css files, should that be somewhere else like my theme location? Why would that matter if the custom css is being pointed to in global settings, especially if I had not made changes. I hope I have given enough information to get some help.

Comments
Firefox/firebug
I'm not sure on the specifics, but FF's firebug is your best friend for troubleshooting things like css definition load-order and paths. First be sure to disable css compression and then load a page with Firebug. It will show you load order for css files (sequence matters as later definitions override earlier ones, unless spec'ed with the !important tag) and you'll see exactly which css is called from which path location.
HtH,
Kyle
Is it only in IE?
Probably not the issue here... but just in case... I've seen stuff like this if you're using IE. IE has a nasty bu.. err... feature that limits the number of link statements to 30. It ignores any files after this. Drupal can reach this limit quickly when you use a lot of modules.
The "official" solution is to turn on CSS optimization in Performance. This will combine all your CSS into a single cached file. However, changes to CSS in your theme won't be picked up until you clear the cache.
An alternative I've used it to install the unlimited_css module. This converts the links to @imports which IE doesn't mind. This module lets you update CSS and then do a full refresh (Left Shift/Refresh) to see you change immediately.
IE as well, Firebug
Replying to both replies at once.
As to Firebug, if I am understanding it correctly, from the menus under "css" the drop down menu shows the loaded css from these areas; node, system, user, nice_menus, and theme. I assume they loaded from top to bottom in the list in that order. Which means the theme css loads last overriding all previous unless !important is specified. I have css compression turned off and have have cleared the cache a number of times.
It happens in IE8 as well, but I typically work with FF. Currently Nice_Menus is the only contributed module I have installed. According to the firebug list, only nine css files have loaded.
Wait Wait I fixed one of the issues, when I renamed the "nice_menus_default.css" file to "my_nice_menus.css" and pointed to it in themes>configure>global settings, it worked with Garland because I also changed the name on the first line of the css file at /* $Id: my_nice_menus.css,*/to match my css file name. They never mentioned that in the documentation, maybe everyone else knows to do that.
Now I am trying to make the theme I want to use to play nice. Maybe I can just copy all the custom nice menu file in the bottom of my theme css file, or stick !important on all the css areas in my custom "my_nice_menus.css".
sample path
For anyone else having trouble with this, a sample path is:
sites/all/themes/your-theme-name/css/your-file-name.css
or if you want to keep it in the modules folder:
sites/all/modules/nice_menus/css/nice_menus_default.css