Maybe this fixes it?

Author: mxstbr

packages/preset-polaris/package.json

@@ -12,7 +12,7 @@
     "watch": "microbundle watch --no-compress"
   },
   "dependencies": {
-    "@theme-ui/preset-base": "^0.2.44"
+    "@theme-ui/preset-base": "^0.3.0"
   },
   "publishConfig": {
     "access": "public"

yarn.lock

@@ -3335,11 +3335,6 @@
     "@testing-library/dom" "^6.11.0"
     "@types/testing-library__react" "^9.1.2"
 
-"@theme-ui/preset-base@^0.2.44":
-  version "0.2.44"
-  resolved "https://registry.yarnpkg.com/@theme-ui/preset-base/-/preset-base-0.2.44.tgz#ae8e938288a534f129111a56b1971e1cc12ddc9d"
-  integrity sha512-g2BaHKz5/32wHIFoRifHxYikBJICqYx+weypA/zwz+d7rZTE9q+EgTvlFSdVlf1HtR2a3uLB1lp4se0tlI2zwg==
-
 "@types/agent-base@^4.2.0":
   version "4.2.0"
   resolved "https://registry.yarnpkg.com/@types/agent-base/-/agent-base-4.2.0.tgz#00644e8b395b40e1bf50aaf1d22cabc1200d5051"