style(docs): use simpler css selectors

Author: camerondubas

tests/dummy/app/components/code-snippet/template.hbs

@@ -1 +1,5 @@
-<CodeBlock @code={{this.snippet.source}} @language={{this.language}} />
+<CodeBlock
+  @code={{this.snippet.source}}
+  @language={{this.language}}
+  class="code-snippet"
+/>

tests/dummy/app/styles/app.scss

@@ -201,19 +201,15 @@ th, td {
   color: #bbb;
 }
 
-pre > code {
+.code-snippet {
   border: none;
   margin: 0;
   padding: 0 .5rem;
 }
 
-:not(pre) > code[class*="language-"], pre[class*="language-"] {
-  background-color: #f8f8f8;
-}
-
 .code-template-toggle {
   position: relative;
-  background-color: #f8f8f8;
+  background-color: #f5f2f0;
   pre {
     margin: 0 !important;
   }