Quoting from themod_rewrite documentation:

"You will almost always want to use[R]in conjunction with[L](that is, use[R,L]) because on its own, the[R]flag prependshttp://thishost[:thisport]to the URI, but then passes this on to the next rule in the ruleset, which can often result in 'Invalid URI in request' warnings."

In fact, I'd recommend using[NS,L,R=code]for external … Read more

I cannot see how node red and CSS pages are connected. Did I miss something? Can you give us some more context?

You can create a simple HTTP-In->Template->Http-Response flow that serves up your CSS content. The HTTP-In node will define the URL the content is served from, the Template node will contain the CSS itself