Rollup merge of rust-lang#83157 - nagisa:nagisa/portability-background, r=GuillaumeGomez

No background for code in portability snippets

This better matches the appearance of this kind of snippet in the full
item view and is less jarring to read due to repeated
foreground-background changes.

![Listing of items in a module with some portability snippets attached to some of the items (light theme). The portability snippet has a light blue background and all of the text in it, monospace or not, is the same colour – black](https://user-images.githubusercontent.com/679122/111196363-1900f500-85b5-11eb-8f97-e283c59002a4.png)
![Listing of items in a module with some portability snippets attached to some of the items (dark theme). The portability snippet has a light blue background and all of the text in it, monospace or not, is the same colour – black](https://user-images.githubusercontent.com/679122/111196366-19998b80-85b5-11eb-9914-4d14d9d13ed3.png)

There should be no observable changes to the ayu theme.

Author: JohnTitor

src/librustdoc/html/static/themes/ayu.css

@@ -266,7 +266,7 @@ a {
 
 .stab.portability > code {
 	color: #e6e1cf;
-	background-color: transparent;
+	background: none;
 }
 
 #help > div {

src/librustdoc/html/static/themes/dark.css

@@ -222,10 +222,7 @@ a.test-arrow {
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; color: #2f2f2f; }
 .stab.deprecated { background: #F3DFFF; border-color: #7F0087; color: #2f2f2f; }
 .stab.portability { background: #C4ECFF; border-color: #7BA5DB; color: #2f2f2f; }
-
-.stab.portability > code {
-	color: #ddd;
-}
+.stab.portability > code { background: none; }
 
 #help > div {
 	background: #4d4d4d;

src/librustdoc/html/static/themes/light.css

@@ -220,10 +220,7 @@ a.test-arrow {
 .stab.unstable { background: #FFF5D6; border-color: #FFC600; }
 .stab.deprecated { background: #F3DFFF; border-color: #7F0087; }
 .stab.portability { background: #C4ECFF; border-color: #7BA5DB; }
-
-.stab.portability > code {
-	color: #000;
-}
+.stab.portability > code { background: none; }
 
 #help > div {
 	background: #e9e9e9;