Bug in theme variables passed to grid instance

Hi all,

Some theme variables do not work when passed by `grid.applyTheme()` on a grid instance.

Here are the one I tested:

```JS
HyperGrid.applyTheme({
hoverCellHighlight: {
enabled: true,
backgroundColor: "red", // $purple-100
},
hoverColumnHighlight: {
enabled: false,
backgroundColor: "blue",
},
hoverRowHighlight: {
enabled: false,
backgroundColor: "green",
},
vScrollbarClassPrefix: "scroll",
hScrollbarClassPrefix: "scroll",
});
```
On the other hand they do work when passed as a global theme with `Hypergrid.applyTheme()`.