demos: invisible radios and buttons in dark mode (#4572)

Due to a hard-coded background in our button and radio demo, the buttons and radios were invisible in dark mode.

This is not really helpful when testing buttons or radios against dark themes.

Author: devversion

src/demo-app/button/button-demo.scss

@@ -8,12 +8,11 @@
   section {
     display: flex;
     align-items: center;
-    background-color: #f7f7f7;
     margin: 8px;
   }
 
   p {
-    padding:5px 15px;
+    padding: 5px 15px;
   }
 }
 

src/demo-app/radio/radio-demo.scss

@@ -4,7 +4,6 @@
 }
 
 .demo-section {
-  background-color: #f7f7f7;
   margin: 8px;
   padding: 16px;