diff --git a/packages/@vuepress/theme-default/styles/mobile.styl b/packages/@vuepress/theme-default/styles/mobile.styl
index f5bd32739d..96b19a11ec 100644
--- a/packages/@vuepress/theme-default/styles/mobile.styl
+++ b/packages/@vuepress/theme-default/styles/mobile.styl
@@ -1,6 +1,6 @@
 @require './config'
 
-$mobileSidebarWidth = $sidebarWidth * 0.82
+$mobileSidebarWidth ?= $sidebarWidth * 0.82
 
 // narrow desktop / iPad
 @media (max-width: $MQNarrow)