You can change the \webroot\css\styles_custom_v2.css file to change the appearance of your PRTG installation. You can open the development tools by clicking on F12 within your browser to find the associated .css class. The changes will be directly applied after clearing the browser's cache.
Please note that our support cannot provide extended support with examples for customizations.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.