Rollup merge of rust-lang#75881 - pickfire:patch-5, r=GuillaumeGomez

Expand rustdoc theme chooser x padding

![image](https://user-images.githubusercontent.com/4687791/91057476-d0eea500-e659-11ea-8c9a-e44db937da89.png)
![image](https://user-images.githubusercontent.com/4687791/91057530-e368de80-e659-11ea-9298-fbb00006d91f.png)

But I still think there is room for improvement considering mdbook.

![image](https://user-images.githubusercontent.com/4687791/91057583-f7acdb80-e659-11ea-9dc5-317caed92bc5.png)

CC @GuillaumeGomez @jyn514

src/librustdoc/html/static/rustdoc.css

@@ -1290,7 +1290,7 @@ h4 > .notable-traits {
 #theme-choices > button {
 	border: none;
 	width: 100%;
-	padding: 4px;
+	padding: 4px 8px;
 	text-align: center;
 	background: rgba(0,0,0,0);
 }