Changing heatmap cell? styles on hover

So I have the following codepen example:

Basically, you can see I have a small xgap providing a “border” between cells.

What i’d like to do is either make that a bit larger, and/or provide some kind of “hover state change” (preferably with CSS) on hover.

With plain D3 this is easy, but of course, creating the entire chart with plain D3 is much more verbose than plotlyjs.