tweak code tags in markdown (go-gitea#11000)

- remove whitespace before and after
- increase horizontal padding
- make blocks more apparant on arc-green

Author: silverwind

web_src/less/_markdown.less

@@ -371,21 +371,13 @@
 
     code,
     tt {
-        padding: .2em 0;
+        padding: .2em .3em;
         margin: 0;
         font-size: 85%;
         background-color: rgba(0, 0, 0, .04);
         border-radius: 3px;
     }
 
-    code:before,
-    code:after,
-    tt:before,
-    tt:after {
-        letter-spacing: -.2em;
-        content: "\00a0";
-    }
-
     code br,
     tt br {
         display: none;

web_src/less/themes/theme-arc-green.less

@@ -1584,6 +1584,11 @@ body .xdsoft_datetimepicker {
     border-left-color: #888;
 }
 
+.markdown:not(code) code,
+.markdown:not(code) tt {
+    background-color: rgba(0, 0, 0, .12);
+}
+
 footer .container .links > * {
     border-left-color: #888;
 }