fix: arrow label dark mode color (#1733)

This PR fixes the issue where the arrow label was black in dark mode
instead of white.

light mode (normal):
<img width="600" alt="Screenshot 2023-07-12 at 00 21 35"
src="https://github.com/tldraw/tldraw/assets/5616556/2ac5e951-7777-4e96-9dde-a410a754aa01">

dark mode (before):
<img width="600" alt="Screenshot 2023-07-12 at 00 21 52"
src="https://github.com/tldraw/tldraw/assets/5616556/c67c3309-2847-4499-b4ef-6ba686e1382e">

dark mode (after):
<img width="600" alt="Screenshot 2023-07-12 at 00 21 44"
src="https://github.com/tldraw/tldraw/assets/5616556/244987c0-4570-410b-8975-43db47b786fa">

Fixes #1716 

### Change Type

- [x] `patch` — Bug fix
- [ ] `minor` — New feature
- [ ] `major` — Breaking change
- [ ] `dependencies` — Changes to package dependencies[^1]
- [ ] `documentation` — Changes to the documentation only[^2]
- [ ] `tests` — Changes to any test code only[^2]
- [ ] `internal` — Any other changes that don't affect the published
package[^2]
- [ ] I don't know

[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version

### Test Plan

1. Add an arrow with label
2. Switch between dark and light mode and notice that the label color
adapts correctly

- [ ] Unit Tests
- [ ] End to end tests

### Release Notes

- fixed arrow label dark mode color

Co-authored-by: Steve Ruiz <steveruizok@gmail.com>

packages/tldraw/src/lib/shapes/arrow/ArrowShapeUtil.tsx

@@ -732,7 +732,7 @@ export class ArrowShapeUtil extends ShapeUtil<TLArrowShape> {
 					size={shape.props.size}
 					position={info.middle}
 					width={labelSize?.w ?? 0}
-					labelColor={shape.props.labelColor}
+					labelColor={theme[shape.props.labelColor].solid}
 				/>
 			</>
 		)