docs: add frontmatter defaults and enrich theming

Update Tera link to canonical URL, add authoring syntax
pointer. Add Default column to frontmatter fields table
with source-verified values. Enrich Theme System section
with core CSS variables table and customization pattern.

docs/content/configuration.md

@@ -88,18 +88,18 @@ link_to: https://... # External link (for project cards)
 
 ### Frontmatter Fields
 
-| Field          | Type    | Description                                    |
-| -------------- | ------- | ---------------------------------------------- |
-| `title`        | string  | Page title (required)                          |
-| `description`  | string  | Meta description                               |
-| `date`         | string  | Publication date (YYYY-MM-DD)                  |
-| `weight`       | integer | Sort order (lower = first, default 50)         |
-| `nav_label`    | string  | Override navigation label (defaults to title)  |
-| `section_type` | string  | Template dispatch (e.g., "blog", "projects")   |
-| `template`     | string  | Custom template path                           |
-| `toc`          | boolean | Enable/disable table of contents for this page |
-| `link_to`      | string  | External URL (renders as link instead of page) |
-| `tags`         | list    | Tags for categorization                        |
+| Field          | Type    | Default        | Description                                    |
+| -------------- | ------- | -------------- | ---------------------------------------------- |
+| `title`        | string  | _(required)_   | Page title                                     |
+| `description`  | string  | _(none)_       | Meta description                               |
+| `date`         | string  | _(none)_       | Publication date (YYYY-MM-DD)                  |
+| `weight`       | integer | `50`           | Sort order (lower = first)                     |
+| `nav_label`    | string  | title          | Override navigation label                      |
+| `section_type` | string  | directory name | Template dispatch (e.g., "blog", "projects")   |
+| `template`     | string  | _(none)_       | Custom template name                           |
+| `toc`          | boolean | global setting | Enable/disable table of contents for this page |
+| `link_to`      | string  | _(none)_       | External URL (renders as link instead of page) |
+| `tags`         | list    | `[]`           | Tags for categorization                        |
 
 ### Section Types
 

docs/content/features/syntax-highlighting.md

@@ -205,6 +205,36 @@ sukr includes several themes in the `themes/` directory:
 
 Copy the theme files to your project and import as shown above.
 
+### Core Variables
+
+All themes define these CSS custom properties in `:root`:
+
+| Variable        | Description            |
+| --------------- | ---------------------- |
+| `--hl-keyword`  | Keywords, control flow |
+| `--hl-string`   | String literals        |
+| `--hl-function` | Function names         |
+| `--hl-comment`  | Comments               |
+| `--hl-type`     | Type names             |
+| `--hl-number`   | Numeric literals       |
+| `--hl-variable` | Variables              |
+| `--hl-operator` | Operators              |
+
+### Customizing a Theme
+
+Import a theme, then override specific variables in your own CSS:
+
+```css
+@import "themes/dracula.css";
+
+/* Override just the keyword color */
+:root {
+  --hl-keyword: #e879f9;
+}
+```
+
+Changing a variable updates every scope that references it.
+
 ### Theme Structure
 
 Themes use CSS custom properties for easy customization:

docs/content/features/templates.md

@@ -4,7 +4,7 @@ description: Customizable templates without recompilation
 weight: 1
 ---
 
-sukr uses [Tera](https://tera.netlify.app/), a Jinja2-like templating engine. Templates are loaded at runtime, so you can modify them without recompiling sukr.
+sukr uses [Tera](https://keats.github.io/tera/), a Jinja2-like templating engine. Templates are loaded at runtime, so you can modify them without recompiling sukr. See the [Tera documentation](https://keats.github.io/tera/docs/) for template authoring syntax (filters, blocks, inheritance).
 
 ## Template Directory Structure
 

docs/plans/documentation-overhaul.md

@@ -73,9 +73,9 @@ None. All CHALLENGE questions resolved.
    - [x] Rewrite `_index.md` as explanation quadrant (what sukr is, why it exists, link to tutorial)
    - [x] Slim `features/sections.md` to reference-only (section_type field, template dispatch)
    - [x] Move explanation content from `sections.md` into `content-organization.md`
-   - [ ] Add link to upstream Tera docs in `features/templates.md`
-   - [ ] Add defaults column to frontmatter table in `configuration.md`
-   - [ ] Add "Theming" section to `syntax-highlighting.md` (choosing/customizing themes)
+   - [x] Add link to upstream Tera docs in `features/templates.md`
+   - [x] Add defaults column to frontmatter table in `configuration.md`
+   - [x] Add "Theming" section to `syntax-highlighting.md` (choosing/customizing themes)
 
 3. **Phase 3: Fill Content Gaps** — cover Deploy and Customize stages of user journey
    - [ ] Create `docs/content/deployment.md` (how-to: generic static host deployment + platform links)