#7657 Fix dark-mode contrast in foreach documentation examples

The foreach documentation uses fixed light backgrounds and black grid borders, leaving the todo text unreadable and number-cell borders difficult to see in dark mode. Use theme colors for the backgrounds, text, and borders so both examples follow the active color mode.

Validation

• Built and inspected the production docs preview in dark mode.
• Existing foreach docgen regression check passes (test_docgen_double_eval.py -k foreach).
• Ruff lint, formatting, and git diff --check pass.

Before and after

Before	After

Closes ENG-13556.