diff --git a/docs/claude-design.md b/docs/claude-design.md
index faf62ad..f66c70c 100644
--- a/docs/claude-design.md
+++ b/docs/claude-design.md
@@ -102,6 +102,19 @@ When `nav-background` and `accent` share the same hue (or are very close),
 active nav links become invisible — the coloured text disappears into a
 coloured background.
 
+The nav colour keys also control the **site name, site description, and
+dark/light mode toggle** — all three live inside the sidebar and inherit from
+the same variables. On muted or neutral nav backgrounds the content-area
+fallbacks (`text`, `text-muted`) are fine. On any saturated or bold nav
+background the contrast between `text` and `text-muted` is likely too low for
+these elements to remain legible, so all three nav colour keys must be set
+explicitly.
+
+**Rule of thumb:** if `nav-background` has a saturation above roughly 20 % or
+a lightness below 30 % (dark sidebar) or above 85 % (near-white sidebar that
+differs noticeably from the page background), set `nav-link`,
+`nav-link-active`, and `nav-section-heading` explicitly for that mode.
+
 **Always set all three nav colour keys explicitly** whenever `nav-background`
 is anything other than a neutral near-white (light) or near-black (dark).