Restyle blog with deep-ocean dark/underwater theme

[Loyale]

Summary

• Flip the palette from a light deep-ocean scheme to a dark underwater one. New tokens for the water column (abyss / trench / deep / current / kelp), cool-tinted type (foam / mist / muted), coral links, and a bioluminescent mint accent for the brand and headings.
• Body uses a fixed-viewport vertical gradient roughly following real ocean light attenuation — sunlit teal at top → mesopelagic blue → bathypelagic indigo → abyssal near-black at the bottom — with a faint surface-light radial wash. background-attachment: fixed keeps the bottom of the screen always the deepest, independent of scroll.
• Cleaner layout: narrower 980px container, articles rendered as subtly bordered cards with soft shadow, refined uppercase navbar with bioluminescent hover/active states, pill-shaped tags and buttons, glow on focus.
• Switched CODE_COLOR_SCHEME to monokai and styled .highlight / pre as a dark IDE-style well with a teal border and line-number gutter.
• Jupyter notebook support for both the classic (.input_area, .output, div.prompt) and JupyterLab (.jp-InputArea, .jp-OutputArea, .jp-RenderedHTMLCommon) class trees: cyan input prompts, coral output prompts, translucent teal dataframe headers with zebra rows, coral-tinted stderr cells, and a light backing for matplotlib/SVG outputs so transparent figures stay legible.
• THEME_COLOR updated to #04161d for the mobile browser chrome.

Test plan

• nikola build succeeds with no template/CSS errors
• nikola serve -b — verify home, archive, tags, single post, and About page render in the new theme
• Confirm the body gradient transitions smoothly top→bottom and stays fixed while scrolling
• Verify a Markdown post with fenced code blocks renders cleanly in monokai on the dark well
• Verify a sample Jupyter notebook post renders inputs, outputs, dataframes, and matplotlib figures legibly
• Check responsive header / collapsed navbar on a narrow viewport
• Verify Giscus comments section visually integrates with the dark theme

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