fix: mark inputs of styling plugin as required

While this cannot ensure that the value is valid, it should prevent
accidental saving of an empty field, which crashes the less compiler.

Fixes #2246

lib/plugins/styling/admin.php

@@ -86,7 +86,7 @@ class admin_plugin_styling extends DokuWiki_Admin_Plugin
                 echo '<tr>';
                 echo '<td><label for="tpl__'.hsc($key).'">'.$name.'</label></td>';
                 echo '<td><input type="'.$this->colorType($value).'" name="tpl['.hsc($key).']" id="tpl__'.hsc($key).'"
-                    value="'.hsc($this->colorValue($value)).'" dir="ltr" /></td>';
+                    value="'.hsc($this->colorValue($value)).'" dir="ltr" required="required"/></td>';
                 echo '</tr>';
             }
             echo '</tbody></table>';