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 @@ public function form()
                 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>';