chore(@theme-ui/color): move core to deps

Author: hasparus

packages/color/package.json

@@ -10,12 +10,10 @@
     "watch": "microbundle watch --no-compress"
   },
   "dependencies": {
+    "@theme-ui/core": "^0.3.1",
     "@theme-ui/css": "^0.3.1",
     "polished": "^3.4.1"
   },
-  "peerDependencies": {
-    "@theme-ui/core": "^0.3.1"
-  },
   "publishConfig": {
     "access": "public"
   },