[Discover][Borealis] Fix DocViewer cell color (elastic#202918)

- Closes elastic#202043

## Summary

This PR fixes text color in DocViewer in the light Borealis theme.

Before:
<img width="400" alt="390511364-e73c9c18-a40a-4026-8a94-e24212f19ca4"
src="https://github.com/user-attachments/assets/ecb12c37-3dc3-4163-b9d1-beaebf914faa">


After:
<img width="597" alt="Screenshot 2024-12-09 at 16 12 08"
src="https://github.com/user-attachments/assets/ea839509-6732-426f-b048-6e73fc2270a1">


### Testing

* Add the following to your `kibana.dev.yml`
```
uiSettings.experimental.themeSwitcherEnabled: true
uiSettings.overrides.theme:name: borealis
```
* Start Kibana with `KBN_OPTIMIZER_THEMES="experimental" yarn start`

src/plugins/unified_doc_viewer/public/components/doc_viewer_table/table.scss

@@ -3,7 +3,6 @@
   word-wrap: break-word;
   white-space: pre-wrap;
   line-height: $euiLineHeight;
-  color: $euiColorFullShade;
   vertical-align: top;
 
   &--highlighted {