Remove Extra Padding on Highlighted search results (readthedocs#860)

* Remove Extra Padding on Highlighted search results

There are some cases where the highlighted CSS class creates overlapping text, for example on [Rendering » Shader Nodes » Texture](https://docs.blender.org/manual/en/dev/render/shader_nodes/textures/index.html?highlight=texture) when highlighting "texture". 

This issue is exacerbated by some custom styling rules that we have set, however, this issue is still visible in the stock theme.

* Address Review Comments

- Use constant instead of a magic number
- Remove Padding

src/sass/_theme_rst.sass

@@ -223,9 +223,9 @@
   // Sphinx can highlight searched text with ?highlighted=searchterm
   .highlighted
     background: $highlight-color
-    display: inline-block
+    box-shadow: 0 0 0 $base-line-height / 12 $highlight-color
+    display: inline
     font-weight: bold
-    padding: 0 $base-line-height / 4
 
   // These are the little citation links [1] that show up within paragraphs.
   .footnote-reference, .citation-reference