adds docs for theming (#66)

Aligns with atuinsh/atuin#2236 to explain the
usage of themes.

TODO: Markdown needs to be checked, but here for feedback initially.

More information is available at that PR.

astro.config.mjs

@@ -48,6 +48,7 @@ export default defineConfig({
             { label: 'Import existing history', link: '/guide/import' },
             { label: 'Basic usage', link: '/guide/basic-usage' },
             { label: 'Syncing dotfiles', link: '/guide/dotfiles' },
+            { label: 'Theming', link: '/guide/theming' },
           ],
         },
         {

src/content/docs/configuration/config.mdx

@@ -623,3 +623,48 @@ The port to use for client -> daemon communication. Only used on non-unix system
 ```toml
 tcp_port = 8889
 ```
+
+## theme
+Atuin version: >= 18.4
+
+The theme to use for showing the terminal interface.
+
+```toml
+[theme]
+name = ""
+debug = false
+max_depth = 10
+```
+
+### `name`
+Default: `""`
+
+A theme name that must be present as a built-in (an empty string for the default,
+`autumn` or `marine`), or found in the themes directory, with the suffix `.toml`.
+By default this is `~/.config/atuin/themes/` but can be overridden with the
+`ATUIN_THEME_DIR` environment variable.
+
+```toml
+name = "my-theme"
+```
+
+### `debug`
+Default: `false`
+
+Output information about why a theme will not load. Independent from other log
+levels as it can cause data from the theme file to be printed unfiltered to the
+terminal.
+
+```toml
+debug = false
+```
+
+### `max_depth`
+Default: 10
+
+Number of levels of "parenthood" that will be traversed for a theme. This should not
+need to be added in or changed in normal usage.
+
+```toml
+max_depth = 10
+```

src/content/docs/guide/theming.mdx

@@ -0,0 +1,111 @@
+---
+title: Theming
+---
+
+For terminal interface customization, Atuin supports user and built-in color themes.
+
+Atuin ships with only a couple of built-in alternative themes, but more can be added via TOML files.
+
+# Required config
+
+The following is required in your config file (`~/.config/atuin/config.toml`)
+
+```
+[theme]
+name = "THEMENAME"
+```
+
+Where `THEMENAME` is a known theme. The following themes are available out-of-the-box:
+
+* default theme (can be explicitly referenced with an empty name `""`)
+* `autumn` theme
+* `marine` theme
+
+These are present to ensure users and developers can try out theming, but in general, you
+will need to download themes or make your own.
+
+If you are writing your own themes, you can add the following line to get additional output:
+
+```
+debug = true
+```
+
+to the same config block. This will print out any color names that cannot be parsed from
+the requested theme.
+
+A final optional setting is available:
+
+```
+max_depth: 10
+```
+
+which sets the maximum levels of theme parents to traverse. This should not need to be
+explicitly added in normal use.
+
+# Usage
+
+## Theme structure
+
+Themes are maps from *Meanings*, describing the developer's intentions,
+to (at present) colors. In future, this may be expanded to allow richer style support.
+
+*Meanings* are from an enum with the following values:
+
+* `AlertInfo`: alerting the user at an INFO level
+* `AlertWarn`: alerting the user at a WARN level
+* `AlertError`: alerting the user at an ERROR level
+* `Annotation`: less-critical, supporting text
+* `Base`: default foreground color
+* `Guidance`: instructing the user as help or context
+* `Important`: drawing the user's attention to information
+* `Title`: titling a section or view
+
+These may expand over time as they are added to Atuin's codebase, but Atuin
+should have fallbacks added for any new *Meanings* so that, whether themes limit to
+the present list or take advantage of new *Meanings* in future, they should
+keep working sensibly.
+
+**Note for Atuin contributors**: please do identify and, where appropriate during your own
+PRs, extend the Meanings enum if needed (along with a fallback Meaning!).
+
+## Theme creation
+
+When a theme name is read but not yet loaded, Atuin will look for it in the folder
+`~/.config/atuin/themes/` unless overridden by the `ATUIN_THEME_DIR` environment
+variable. It will attempt to open a file of name `THEMENAME.toml` and read it as a
+map from *Meanings* to colors.
+
+Colors may be specified either as names from the [palette](https://ogeon.github.io/docs/palette/master/palette/named/index.html)
+crate in lowercase, or as six-character hex codes, prefixed with `#`. For example,
+the following are valid color names:
+
+* `#ff0088`
+* `teal`
+* `powderblue`
+
+A theme file, say `my-theme.toml` can then be built up, such as:
+
+```toml
+[theme]
+name = "my-theme"
+parent = "autumn"
+
+[colors]
+AlertInto = "green"
+Guidance = "#888844"
+
+```
+
+where not all of the *Meanings* need to be explicitly defined. If they are absent,
+then the color will be chosen from the parent theme, if one is defined, or if that
+key is missing in the `theme` block, from the default theme.
+
+This theme file should be moved to `~/.config/atuin/themes/my-theme.toml` and the
+following added to `~/.config/atuin/config.toml`:
+
+```
+[theme]
+name = "my-theme"
+```
+
+When you next run Atuin, your theme should be applied.