This PR flattens the color theme object and introduces a new helper, `getColorValue`, that helps pull a color from the theme. This should actually make #6462 (and other color-related work) much more practical.

## Flattening `DefaultColorThemePalette`

This PR flattens the `DefaultColorThemePalette`, removing the nested properties and renaming them instead.

```ts
theme.black.frame.fill
```

Becomes

```ts
theme.black.frameFill
```

## `getColorValue`

This pull request standardizes and simplifies how color values are accessed throughout the codebase by introducing and consistently using the new `getColorValue` utility function.

```ts
theme.black.frameFill
```

Becomes

```ts
getColorValue(theme, 'black', 'frameFill')
```

### Change type

- [x] `api`

### Test plan

- [ ] Unit tests (if present)
- [ ] End to end tests (if present)

### Release notes

- Flattened the color theme object and introduced a `getColorValue` helper for more consistent color access.

### API changes
- Changes to the `DefaultColorThemePalette` object. Any color customization may need to be refactored.