fix(theme): mobile navbar & skipToContent should cover announcementBar (

#8163)

Co-authored-by: sebastienlorber <lorber.sebastien@gmail.com>

packages/docusaurus-theme-classic/src/theme/AnnouncementBar/styles.module.css

@@ -15,8 +15,16 @@
   height: var(--docusaurus-announcement-bar-height);
   background-color: var(--ifm-color-white);
   color: var(--ifm-color-black);
+
+  /*
+  Unfortunately we can't make announcement bar render above the navbar
+  IE need to use border-bottom instead of shadow
+  See https://github.com/facebookincubator/infima/issues/275
+
   box-shadow: var(--ifm-global-shadow-lw);
-  z-index: calc(var(--ifm-z-index-fixed) + 1); /* just above the navbar */
+  z-index: calc(var(--ifm-z-index-fixed) + 1);
+  */
+  border-bottom: 1px solid var(--ifm-color-emphasis-100);
 }
 
 html[data-announcement-bar-initially-dismissed='true'] .announcementBar {