doc: Fix CSS issue with keyboard shortcuts on dark theme

Fixes CSS styling issue causing keyboard shortcuts to sometimes not be
visible (white text on white background) using dark theme.
Fixes #55126.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>
diff --git a/doc/_static/css/custom.css b/doc/_static/css/custom.css
index 45e4887..da9f379 100644
--- a/doc/_static/css/custom.css
+++ b/doc/_static/css/custom.css
@@ -545,7 +545,9 @@
 }
 
 /* Keyboard shortcuts tweaks */
-kbd, .kbd {
+kbd, .kbd,
+.rst-content :not(dl.option-list) > :not(dt):not(kbd):not(.kbd) > kbd,
+.rst-content :not(dl.option-list) > :not(dt):not(kbd):not(.kbd) > .kbd {
     background-color: var(--kbd-background-color);
     border: 1px solid var(--kbd-outline-color);
     border-radius: 3px;