Fix tailwind setup

- Adopted postcss and tailwind config to new version
- renamed colors bluegray->slate etc to not have custom
  definitions (hope to reduce migration next time)
- tailwind now doesn't build the complete css anymore, so the
  `dev-ui-build` script must compile both

project/StylesPlugin.scala

@@ -96,7 +96,7 @@ object StylesPlugin extends AutoPlugin {
       Seq(
         npx,
         "postcss",
-        s"$inDir/*.css",
+        s"$inDir/index.css",
         "-o",
         target.absolutePath,
         "--env",