Fix Nav Buttons Size (#1067)

Currently, Nav buttons to expand the menu are a bit small and do not match the link text.
This makes the font size and line-height match between the text and icon.

src/sass/_theme_layout.sass

@@ -70,8 +70,7 @@ html
       margin-left: -1.2em
       @extend .fa
       @extend .fa-plus-square-o
-      font-size: .8em
-      line-height: 1.6em
+      line-height: 18px
       color: darken($menu-link-medium, 20%)
 
   // On state for the first level
@@ -92,8 +91,7 @@ html
       @extend .fa
       @extend .fa-minus-square-o
       display: block
-      font-size: .8em
-      line-height: 1.6em
+      line-height: 18px
       color: darken($menu-link-medium, 30%)
 
   li.toctree-l1.current > a