webconfig: Use flexbox for the colorschemes

This had a classic float:left layout, which led to awkward gaps and
stuff.

Since what we want here is basically 100% exactly a flexbox, just use that.

Note: No flexbox for the prompts, atm, because having multiple of
those next to each other looks a bit weird.

share/tools/web_config/fishconfig.css

@@ -447,8 +447,12 @@ code {
     bottom: 0px;
 }
 
+.color_scheme_choices_list {
+    display: flex;
+    flex-wrap: wrap;
+}
+
 .color_scheme_choice_container {
-    float: left;
     padding: 5px;
 }