It's this: https://tabler.io/admin-template

Still highly unstable between versions, but for being free, it looks really good and is very flexible. I've been pleased with it for this project.

Seems to me a lot of what Tabler does you could do easily today in "vanilla" CSS Grid, which is also even more flexible. (I have done some very dynamic dashboards with simple CSS class changes and CSS Grid. You can do so much just changing grid-template-areas, and I really like its feel as an ASCII diagram of the layout that you want so I think it's also easy to scan a lot of layout variations in a CSS file to see which one is which.)

At this point I've been ditching almost every other type of layout (Bootstrap's Grid, Flexbox, etc) in most places for just "vanilla" CSS Grid and I think it is fantastic. I think it is well worth learning if you haven't yet.