tweak code tags in markdown (#11000)

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

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;
 }