Two small styling fixes

* Use font from variables
* No need to load bourbon twice

sass/_core.scss

@@ -91,7 +91,7 @@ body.converse-fullscreen {
     padding-left: env(safe-area-inset-left);
     padding-right: env(safe-area-inset-right);
     color: var(--text-color);
-    font-family: "Helvetica", "Arial", sans-serif;
+    font-family: var(--normal-font);
     font-size: var(--font-size);
     direction: ltr;
     z-index: 1031; // One more than bootstrap navbar

sass/_embedded.scss

-@import "bourbon";
 #conversejs.converse-embedded {
 
     @include box-sizing(border-box);