Use either css mode:
and add color fontification by using code found here,
or inline the css in web:
EDIT after comment from @predicate and @Dan:
Ok, I see. The problem is, that
web-mode detects css based on file extension, which does not work for source blocks. As a workaround you could define a minimal helper mode for each language you use. You would have to call
web-mode to initialize web mode and then change the type by setting buffer local variable
Here an example for a web-css-mode:
(define-derived-mode web-css-mode web-mode "WebCss"
"Major mode for editing web css templates."
(setq web-mode-content-type "css"))
And its usage in org-mode:
Did not try it, but you might also be able to define another engine by inserting
/* -*- engine:php -*- */
(replacing php with the engine you want, see variable
web-mode-engines or topic "Associate an engine" here) in your buffer.