Yes. You can apply as many classes as you wish to a menu item (web page) or module.

For Menu Items

Open up the menu item under Menus.

Click on the Advanced Options tab.

Click on the section Page Display Options.

You will see a text box labeled Page Class. In this text box, you may add your classes. Simply separate them with a SPACE. If your template does not add this class to your page, click here to read the joom3 article on how to edit your template files to enable you to add specific classes to your pages.

For Modules:

Open up the module in question under Extensions -> Module Manager.

Click on Options -> Advanced Options.

Go to the text box labeled Module Class Suffix.

You can type your class name(s) in there, but note how this says "suffix". Joomla will add the term 'custom' to the front of your first class name. If you don't want this to happen, simply add a space before the class name.

Styling these classes:

Now you can target that class within your CSS stylesheet. You can add a specific class name to as many modules or web pages as you wish.