Add css highlighting documentation

This is copied from getzola#913

Co-Authored-By: Uwe Arzt <[email protected]>

docs/content/documentation/content/css-syntax-highlighting.md

@@ -0,0 +1,72 @@
++++
+title = "CSS Syntax Highlighting"
+weight = 80
++++
+
+If you use a highlighting scheme like
+
+```toml
+highlight_theme = "base16-ocean-dark"
+```
+
+for a code block like
+
+````md
+```rs
+let highlight = true;
+```
+````
+
+you get the colors directly encoded in the html file
+
+```html
+<pre style="background-color:#2b303b;">
+    <code>
+        <span style="color:#b48ead;">let</span>
+        <span style="color:#c0c5ce;"> highlight = </span>
+        <span style="color:#d08770;">true</span>
+        <span style="color:#c0c5ce;">;
+    </span>
+  </code>
+</pre>
+```
+
+this is nice, because if everything is inside one file, you get fast
+page loadings. But if you would like to have the user choose a theme from a
+list, or different color schemes for dark/light color schemes, ou need a
+different solution.
+
+If you use the special color scheme
+
+```toml
+highlight_theme = "css"
+```
+
+you get CSS class definitions
+
+```html
+<pre class="code">
+    <code class="code">
+        <span class="source rust">
+            <span class="storage type rust">let</span> highlight
+            <span class="keyword operator assignment rust">=</span>
+            <span class="constant language rust">true</span>
+            <span class="punctuation terminator rust">;</span>
+        </span>
+    </code>
+</pre>
+```
+
+now you can generate and use CSS either manually or with Zola
+
+```toml
+highlighting_themes_css = [
+  { theme = "base16-ocean-dark", filename = "syntax-theme-dark.css" },
+  { theme = "base16-ocean-light", filename = "syntax-theme-light.css" },
+]
+```
+
+```css
+@import url("syntax-theme-dark.css") (prefers-color-scheme: dark);
+@import url("syntax-theme-light.css") (prefers-color-scheme: light);
+```

docs/content/documentation/getting-started/configuration.md

@@ -41,7 +41,17 @@ highlight_code = false
 
 # The theme to use for code highlighting.
 # See below for list of allowed values.
+# This will put the colors of the theme directly in your html.
 highlight_theme = "base16-ocean-dark"
+# For using CSS class definitions in the higlighting, you can use the
+# special theme "css". This is especially useful for dark/light themes,
+# or letting the user decide the higlighting scheme.
+# If the site uses some of the predefined syntax highlighing schemes as
+# CSS, you can let Zola generate the CSS.
+highlighting_themes_css = [
+  { theme = "base16-ocean-dark", filename = "syntax-theme-dark.css" },
+  { theme = "base16-ocean-light", filename = "syntax-theme-light.css" },
+]
 
 # When set to "true", a feed is automatically generated.
 generate_feed = false