Enonic UI: darkmode png

PNG icons are now displayed inverse for darkmode:

<img width="232" height="149" alt="Image" src="https://github.com/user-attachments/assets/c21bff36-ffcb-449d-9af8-a3623ed48da7" />

Also in main tree:

<img width="264" height="207" alt="Image" src="https://github.com/user-attachments/assets/becb8942-d3b8-4e49-b0c3-8870f7ed3073" />