update playground styles

packages/editor/src/main.scss

@@ -1,4 +1,11 @@
-@import 'bootstrap';
+@import '@growi/core-styles/scss/bootstrap/apply';
+
 @import 'react-toastify/scss/main';
 
 @import '@growi/core-styles/scss/helpers/flex-expand';
+
+:root {
+  --font-family-sans-serif: -apple-system, blinkmacsystemfont, 'Hiragino Kaku Gothic ProN', meiryo, sans-serif;
+  --font-family-serif: georgia, 'Times New Roman', times, serif;
+  --font-family-monospace: Menlo, Consolas, DejaVu Sans Mono, monospace;
+}