fix: prevent Django admin dark mode from breaking editor text color

[gghez]

Summary

• When Django admin is in dark mode, color: white on body cascades into the CKEditor content area, making text invisible (white on white).
• Explicitly set color and background on .ck-content using CKEditor's own CSS variables (--ck-color-text and --ck-color-base-background), breaking the inheritance chain.
• Custom dark themes via CKEDITOR_5_CUSTOM_CSS still work — they override the same variables.

Closes #305

Test plan

• Open Django admin with prefers-color-scheme: dark — editor text should be dark on white background
• Switch to light mode — no visual change (was already working)
• If using CKEDITOR_5_CUSTOM_CSS with dark CKEditor variables — editor should render with custom dark theme