themeOverrides with `undefined` values result in Error during render.

Hello,

First of all thanks for developing this project. We use this in production to implement a minimal spreadsheet-like preview and it works great!

We recently bumped the dependency of `@glideapps/glide-data-grid` and `@glideapps/glide-data-grid-source` from `5.3.2` to `6.0.4-alpha24` and got this "crash":

```js
color-parser.js:17 Uncaught TypeError: Cannot read properties of undefined (reading 'toLowerCase')
at parseToRgba (color-parser.js:17:1)
at blend (color-parser.js:67:1)
at mergeAndRealizeTheme (styles.js:109:1)
at data-grid-render.header.js:33:1
at walkColumns (data-grid-render.walk.js:43:1)
at drawGridHeaders (data-grid-render.header.js:22:1)
at drawHeaderTexture (data-grid-render.js:147:1)
at drawGrid (data-grid-render.js:231:1)
at data-grid.js:393:1
at data-grid.js:451:1
```

Which looks to end up at this line:

https://github.com/glideapps/glide-data-grid/blob/0875d78cc41535a39f8b417ff88ce1ebe129c363/packages/core/src/internal/data-grid/color-parser.ts#L23

Our offending code/repro defined a themeOverride like this on the `GridColumn` type:

```js
themeOverride: {
bgCell: visualizeHidden ? 'lightgrey' : undefined,
bgHeader: visualizeHidden ? 'lightgrey' : undefined,
}
```

It's of course quite easy to get around this issue for us by filtering out undefined properties from the `themeOverride`. But I wanted to give you the chance to fix this if you consider it a bug in the new major.

Please close this issue if you believe this is the intended behavior going forwards.