markdown_preview: Improve table elements appearance (#39101)
# How
Eliminate double borders between Markdown rows and cells, restyle
headers relying on background color alteration instead of thicker pixel
border.
Release Notes:
- Improved table elements appearance in Markdown Preview
# Preview
### Before
<img width="1206" height="594" alt="Screenshot 2025-09-29 at 13 28 23"
src="https://github.com/user-attachments/assets/9fe2b8a8-13e1-4052-9e97-34559b44f2d0"
/>
### After
<img width="1206" height="578" alt="Screenshot 2025-09-29 at 13 28 40"
src="https://github.com/user-attachments/assets/0b627ada-f287-436b-9448-92900d4bff59"
/>
632e569c5fBartosz Kaszubowski committed on 9/29/2025, 12:41:41 PM· committed by GitHubparent0c71aa9