[css-writing-modes] Style to accommodate dark mode style sheet.

Author: fantasai

css-writing-modes-3/Overview.bs

@@ -33,6 +33,7 @@ spec:css-text-3; type:dfn; text:character
 <style>
 object { vertical-align: middle; }
 .sidebar { float: right; background: #eee; border: solid gray; margin: 1em; }
+@media (prefer-color-scheme: dark) { .sidebar { background: var(--borderedblock-bg, #eee); } }
 .sidebar .figure { margin: 1em; }
 .sidebar object { margin: 0 auto; display: block; }
 .figurepair { display: table; margin: 1em auto; }

css-writing-modes-4/Overview.bs

@@ -31,6 +31,7 @@ spec:css-text-3; type:dfn; text:character
 <style>
 object { vertical-align: middle; }
 .sidebar { float: right; background: #eee; border: solid gray; margin: 1em; }
+@media (prefer-color-scheme: dark) { .sidebar { background: var(--borderedblock-bg, #eee); } }
 .sidebar .figure { margin: 1em; }
 .sidebar object { margin: 0 auto; display: block; }
 .figurepair { display: table; margin: 1em auto; }