x/website: barely visible image in dark theme

[alexandear]

What did you do?

1. Set the dark appearance in macOS.
2. Open the blog post https://go.dev/blog/llmpowered in the latest Chrome browser.

What did you see happen?

The RAG diagram is barely visible due to the transparent background:

What did you expect to see?

The RAG diagram image has a background. Something like this:

[gopherbot]

Change https://go.dev/cl/613435 mentions this issue: _content/blog/llmpowered: set background for the diagram

[cagedmantis]

cc @golang/tools-team