As software updates are replacing FileRun's files regardless of the fact that they have been customized, there is a way to make CSS modification which will be preserved between software updates.

You can write your custom CSS code inside the file customizables/theme.css and enable it from the control panel under InterfaceBranding, see the Color theme section.

You can also specify the URL of an external CSS file which will get loaded. Simply add the following line inside the configuration file (customizables/config.php, create if doesn't exist, start with <?php on first line):

$config['app']['ui']['custom_css_url'] = '';

Replace the example URL with your own valid URL.