feat: multiple built-in themes

[engineervix]

Summary

Bundle several alternative visual themes in the binary and let users switch between them with a single config key. The current design (warm off-whites + IBM Plex Sans + Lora) becomes the default theme; additional themes offer different colour palettes, typography stacks, or layout density.

Motivation

Kwelea currently has one look. Projects with strong brand identities or different aesthetic preferences have no way to change the overall feel without forking the theme CSS (or waiting for issue #17 to cover all the ground they need). Shipping 2–4 curated built-in themes makes Kwelea useful for a wider range of projects out of the box.

Proposed config key

[theme]
name = "default"   # built-in theme name; defaults to "default"

Candidate themes (starting point for discussion)

Name	Flavour
default	Current design — warm off-whites, IBM Plex Sans + Lora, deep ink dark mode
mono	Monochrome, high-contrast, terminal-inspired
ocean	Cool blue-greens, modern sans-serif throughout
minimal	Maximum whitespace, neutral greys, no colour accents

Names and aesthetics are open for bikeshedding — the point is that the mechanism exists.

Implementation approach

Two reasonable options:

A — CSS custom property overrides per theme

Each theme is a small CSS file that redefines the CSS custom properties declared in theme.css (colours, font stacks, spacing tokens). theme.css stays as the structural base; the theme file is injected after it.

Pros: Small per-theme files; structural CSS (layout, components) is shared.
Cons: Only covers what's expressed as custom properties; deep visual changes (e.g. different typography scale) require expanding the property set.

B — Full CSS replacement per theme

Each theme is a complete standalone CSS file embedded in the binary. The builder writes whichever one the user selects.

Pros: Total freedom per theme.
Cons: Larger binary; duplication of structural CSS across themes.

Option A is the right starting point. The custom property set in theme.css should be audited and expanded to cover everything a theme might want to change.

Notes

• This is a larger effort than issue feat: custom CSS file for user-defined style overrides #17 (custom CSS override). feat: custom CSS file for user-defined style overrides #17 is the right tool for per-project tweaks; this issue is about curated first-party alternatives.
• Themes affect only CSS (and optionally font choices). Templates stay the same across all themes.
• light_code_theme / dark_code_theme remain separate — they control Chroma syntax highlighting independently.
• The dev server (kwelea serve) must pick up a theme change on kwelea.toml edit without a manual restart.