When you're inside a CSS file in NetBeans IDE, press Alt-Insert, and then you can call up this dialog, to insert new CSS rules into the CSS file:
When OK is clicked above, the skeleton of a new CSS rule is inserted into the CSS file.
Next, when you're using your CSS rules, such as below, there are some cool things you can do.
Firstly, press Alt-F7 above, i.e., while you've selected the name of a rule that you've defined, and then a search of all usages is done, same as is done in Java source files and other kinds of files, with the same kind of result:
And then you can click on the items above to jump to the file and the line where they're defined. The above is totally different to a text search, for example, if I have "menu" somewhere in the comments, which is more than likely, that instance of "menu" will not be found since it is not a usage of the "menu" class.
Secondly, when you press Ctrl-R, while the "menu" class above is selected in an HTML file, you call up the in-place rename dialog, just like you would do in a Java source file. Then you can rename "menu", which will also rename all occurrences of "menu", e.g., in the CSS file where that rule is defined.