lightbox.less is not cached

Reproduction steps

1. Visit http://localhost:8080/xwiki/bin/view/Sandbox/
2. open the browser debugger and make sure "disabled cache" is unchecked
3. Click on Sandbox Test Page 1 to go to http://localhost:8080/xwiki/bin/view/Sandbox/TestPage1
4. check the network tab

Expected

http://localhost:8080/xwiki/webjars/wiki%3Axwiki/xwiki-platform-image-lightbox-webjar/17.2.0-SNAPSHOT/less/lightbox.less?evaluate=true is cached

Actual

http://localhost:8080/xwiki/webjars/wiki%3Axwiki/xwiki-platform-image-lightbox-webjar/17.2.0-SNAPSHOT/less/lightbox.less?evaluate=true is not cached

Afaics this is only required for theme variables, which can now be replace with css variables