DEV: Define --footer-nav-height css var (#14008)

DEV: Define --footer-nav-height css var (#14008)

diff --git a/app/assets/stylesheets/common/components/footer-nav.scss b/app/assets/stylesheets/common/components/footer-nav.scss
index 71ed2f9..194fd4b 100644
--- a/app/assets/stylesheets/common/components/footer-nav.scss
+++ b/app/assets/stylesheets/common/components/footer-nav.scss
@@ -2,7 +2,10 @@
 // Footer nav bar (displayed in DiscourseHub app and PWAs)
 // --------------------------------------------------
 
-$footer-nav-height: 49px;
+:root {
+  --footer-nav-height: 49px;
+}
+$footer-nav-height: var(--footer-nav-height);
 
 body.footer-nav-visible {
   #main-outlet {

GitHub sha: 3119b881aa6c297e3fdb281188a1d1b9a5fcd349

This commit appears in #14008 which was approved by CvX. It was merged by markvanlan.