You can not upload a file with a .css extension to the GeoCities server, so you may have concluded that there is no way to use style sheets.
However, you can upload .txt files, and these can be invoked as external style sheets. You will need to compose your style sheet in Notepad and then upload it. Then, you need only refer to it in the HEAD section of your HTML code.
Here is the syntax in the HEAD section of this web page
which refers to the style sheet:
link rel=stylesheet type="text/css" href="http://www.jamesmcquaid.com/style2b.txt"
(you will need to enclose it with greater than and less than symbols).
Click here to see the code in the style sheet.
Click here to an identically coded page using a different style sheet.