fixup! Bug 41916: Letterboxing preferences UI

Bug 42667: Add the "description-deemphasized" class to our preferences.

browser/components/preferences/letterboxing.inc.xhtml

@@ -6,7 +6,7 @@
   data-category="paneGeneral"
 >
   <html:h1 data-l10n-id="letterboxing-header" />
-  <description class="letterboxing-overview">
+  <description class="letterboxing-overview description-deemphasized">
     <html:span
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"
@@ -25,7 +25,7 @@
 >
   <label><html:h2
     data-l10n-id="letterboxing-window-size-header"/></label>
-  <description class="letterboxing-search-overview">
+  <description class="letterboxing-search-overview description-deemphasized">
     <html:span
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"
@@ -50,7 +50,7 @@
   <label><html:h2
     data-l10n-id="letterboxing-alignment-header"
     /></label>
-  <description class="letterboxing-search-overview">
+  <description class="letterboxing-search-overview description-deemphasized">
     <html:span
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"
@@ -99,7 +99,7 @@
 >
   <label class="letterboxing-search-heading"><html:h2
     data-l10n-id="letterboxing-header"/></label>
-  <description class="letterboxing-search-overview">
+  <description class="letterboxing-search-overview description-deemphasized">
     <html:span
       data-l10n-id="letterboxing-overview"
       class="tail-with-learn-more"