diff --git a/README.md b/README.md
index 32ebe07..70096b3 100644
--- a/README.md
+++ b/README.md
@@ -7,6 +7,7 @@ site generator.
 ## Layout
 
 - `site/` — website sources: `calepin.toml` + `.typ` pages
+- `site/themes/yggdrasil/` — local theme extending the bundled `calepin` theme
 - `public/` — generated site (gitignored)
 - `tools/` — pinned `calepin` and `typst` binaries (gitignored, downloaded on demand)
 - `scripts/` — helper scripts used by the Justfile and CI
@@ -59,6 +60,12 @@ Sidebar labels come from the page's `#metadata(...)` line, not from the config.
    top navigation bar. The `label` is required here because the index page's
    metadata title is `Overview`.
 
+Pages inside a category show only that category's sidebar section — the
+local theme (`site/themes/yggdrasil/partials/site-nav-sidebar.html`) filters
+the sidebar to the section containing the current page. Pages outside every
+category (landing page, `404`, standalone pages) fall back to the full
+sidebar.
+
 ## Deployment
 
 `just build` produces a fully static site in `public/`: HTML pages plus
diff --git a/site/calepin.toml b/site/calepin.toml
index 15a4265..fade09e 100644
--- a/site/calepin.toml
+++ b/site/calepin.toml
@@ -1,7 +1,7 @@
 title = "Yggdrasil"
 description = "Personal knowledge base."
 base-url = "https://yggdrasil.kb28.ch"
-theme = "calepin"
+theme = "themes/yggdrasil"
 pdf = false
 minify = true
 search = "pagefind"
diff --git a/site/themes/yggdrasil/partials/site-nav-sidebar.html b/site/themes/yggdrasil/partials/site-nav-sidebar.html
new file mode 100644
index 0000000..5018419
--- /dev/null
+++ b/site/themes/yggdrasil/partials/site-nav-sidebar.html
@@ -0,0 +1,47 @@
+        <aside id="calepin-website-sidebar" class="calepin-website-sidebar">
+            <nav class="calepin-website-mobile-drawer-nav" aria-label="Site navigation">
+                <p><strong>Site</strong></p>
+                <ul>
+                    {% for item in site.menus.main %}
+                    {% include "partials/site-nav-item.html" %}
+                    {% endfor %}
+                    {% for item in site.menus.social %}
+                    {% include "partials/site-nav-item.html" %}
+                    {% endfor %}
+                </ul>
+                <div class="calepin-website-mobile-drawer-actions" aria-label="Site controls">
+                    {% set calepin_theme_toggle_storage_key = "calepin-website-theme" %}
+                    {% set calepin_theme_toggle_param = "theme" %}
+                    {% include "partials/theme-toggle-button.html" %}
+                    {% include "partials/site-language-picker.html" %}
+                    {% if site.page_source or site.page_pdf %}
+                    <select class="calepin-website-view-mode" aria-label="Choose page view" data-calepin-view-mode>
+                        <option value="rendered">HTML</option>
+                        {% if site.page_source %}<option value="source">Source</option>{% endif %}
+                        {% if site.page_pdf %}<option value="pdf">PDF</option>{% endif %}
+                    </select>
+                    {% endif %}
+                </div>
+            </nav>
+            <nav aria-label="Documentation">
+                {#- Scope the sidebar to the current master category: render only
+                    the active section(s). Pages outside every section (landing,
+                    404, standalone pages) fall back to the full sidebar. -#}
+                {% set active_sections = site.sidebar_sections | selectattr("active") | list %}
+                {% if active_sections %}
+                    {% for section in active_sections %}
+                    {% include "partials/site-sidebar-section.html" %}
+                    {% endfor %}
+                {% elif site.sidebar_sections %}
+                    {% for section in site.sidebar_sections %}
+                    {% include "partials/site-sidebar-section.html" %}
+                    {% endfor %}
+                {% elif site.sidebar %}
+                    <ul>
+                        {% for item in site.sidebar %}
+                        {% include "partials/site-sidebar-item.html" %}
+                        {% endfor %}
+                    </ul>
+                {% endif %}
+            </nav>
+        </aside>
diff --git a/site/themes/yggdrasil/theme.toml b/site/themes/yggdrasil/theme.toml
new file mode 100644
index 0000000..4f40ac0
--- /dev/null
+++ b/site/themes/yggdrasil/theme.toml
@@ -0,0 +1 @@
+extends = "calepin"