[spec] Add custom CSS file.

This is required to override CSS flags provided by mdbook. The change
also sets `--content-max-width` to 900px to make tables easier to
navigate on large screens.

Signed-off-by: Miguel Osorio <miguelosorio@google.com>

Author: moidx

docs/book.toml

@@ -10,6 +10,7 @@ command = "mdbook-mermaid"
 
 [output.html]
 additional-js = ["src/third_party/mermaid.min.js", "src/third_party/mermaid-init.js"]
+additional-css = ["theme/custom.css"]
 default-theme = "light"
 
 [output.html.fold]

docs/theme/custom.css

@@ -0,0 +1,7 @@
+:root {
+    --content-max-width: 900px;
+}
+
+table {
+    width: 100%;
+}