unrecognized css stylesheet

I just re-read your post and am wondering if you just need help linking an existing css file to your pages?

To do that, you should be able to...

1. Right-Click in Code View, just before the ending </head> tag
2. Choose CSS Styles > Attach Stylesheet
3. Browse to your CSS file 
4. Verify Link is selected for Add as: 
5. Hit OK

You should also be able to...

1. Click Format in the main menu bar
2. Choose CSS Styles > Attach Stylesheet and follow the same steps above

If the options aren't there (Adobe spent the last 15 years moving things for moving's sake) you can add the link manually in Code View...

1. Just before the ending </head> tag of your page, add <link href="your/stylesheet/path-here.css" rel="stylesheet" type="text/css">

A few things I can think of that might cause a css file to stop working...

Are you working within a Defined Site in DW? DW needs a defined site in order to handle file associations correctly. If you don't have a site defined, internal page links can break easily, especially when uploaded. If your css link starts with "file" inside the page, it would point to an improperly set up site.

It's not inside a .zip archive by chance? When working with html inside a .zip file, internal links won't work until all of the files are first extracted from the archive.

Case structure is important. STYLESHEET.CSS and stylesheet.css are the same on Windows and Mac operating systems. A link to one will go to the other without issue, however when moved to a server they're separate files/file locations. Links within your page must match the case structure of the file exactly when uploaded or they'll stop working.

Validate code and fix all reported errors.

HTML —  https://validator.w3.org/

CSS — https://jigsaw.w3.org/css-validator/

Failing that, post back with a URL to the problem page online.

Yes, CS3 is very outdated and 15 year old code is not fit for use anymore.  You really should be using a modern code editor.  If not Dreamweaver, then something else.

CODE EDITORS:
-- Adobe Dreamweaver CC - https://www.adobe.com/products/dreamweaver.html
-- Atom (free) - https://atom.io/
-- Codespaces (free, browser-based) - https://github.com/features/codespaces
-- Nova (Mac only, formerly called Coda) - https://nova.app/
-- Pinegrow - https://pinegrow.com/
-- Sublime Text - http://www.sublimetext.com/
-- Visual Studio Code (free) - https://code.visualstudio.com/

Read chapters, do code exercises and take quizzes at the end.
- https://www.w3schools.com/html/
- https://www.w3schools.com/css/
- https://www.w3schools.com/js/

Moved from Using The Community forum to Dreamweaver forum.