Add low-contrast, eye-comfort theme options to DevTools

[5344Luc]

Currently, DevTools' dark theme uses highly saturated syntax colors (bright cyan, magenta, green) against a near-black background. Over long debugging sessions this creates harsh contrast that causes eye strain, especially in low-light environments.

Suggestions:

• Desaturated/muted syntax colors instead of neon cyan/magenta/green (similar to how VS Code's "One Dark Pro" tones down saturation while staying readable)
• A "soft dark gray" background option (~#1e1e1e) as an alternative to true black, since pure black increases perceived contrast
• Slightly increased line height/spacing in the JSON/object tree view for easier scanning
• A few curated built-in theme presets (e.g., Solarized Dark, low-contrast) beyond just Light/Dark/Firefox default

Developers spend hours daily in DevTools — small ergonomic improvements here compound significantly over time, the same reason code editors offer many accessible theme choices.

[Jon]

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