fixes #12998 nim doc regression (#13117)

compiler/docgen.nim

@@ -17,7 +17,7 @@ import
   packages/docutils/rst, packages/docutils/rstgen,
   json, xmltree, cgi, trees, types,
   typesrenderer, astalgo, lineinfos, intsets,
-  pathutils, trees
+  pathutils, trees, nimconf
 
 const
   exportSection = skField
@@ -43,6 +43,7 @@ type
     destFile*: AbsoluteFile
     thisDir*: AbsoluteDir
     examples: string
+    wroteCss*: bool
 
   PDoc* = ref TDocumentor ## Alias to type less.
 
@@ -1069,10 +1070,16 @@ proc writeOutput*(d: PDoc, useWarning = false) =
     template outfile: untyped = d.destFile
     #let outfile = getOutFile2(d.conf, shortenDir(d.conf, filename), outExt, "htmldocs")
     createDir(outfile.splitFile.dir)
+    d.conf.outFile = outfile.extractFilename.RelativeFile
     if not writeRope(content, outfile):
       rawMessage(d.conf, if useWarning: warnCannotOpenFile else: errCannotOpenFile,
         outfile.string)
-    d.conf.outFile = outfile.extractFilename.RelativeFile
+    elif not d.wroteCss:
+      let cssSource = $d.conf.getPrefixDir() / "doc" / "nimdoc.css"
+      let cssDest = $d.conf.outDir / "nimdoc.out.css"
+        # renamed to make it easier to use with gitignore in user's repos
+      copyFile(cssSource, cssDest)
+      d.wroteCss = true
 
 proc writeOutputJson*(d: PDoc, useWarning = false) =
   runAllExamples(d)