Follow up to https://github.com/zed-industries/zed/pull/27489

Added this subtle gradient to the right side of the tool card header so
users know there is more content, suggesting it can be scrolled. Also
took the opportunity to extract out commonly used custom colors in all
of these cards into their own functions to ensure consistency.

Here's the final product:


https://github.com/user-attachments/assets/e44150f9-7751-46c7-8790-149b86cc5e0f

Release Notes:

- N/A