--- a/browser/themes/shared/customizableui/customizeMode.inc.css
+++ b/browser/themes/shared/customizableui/customizeMode.inc.css
@@ -24,17 +24,17 @@
   background-image: linear-gradient(var(--toolbar-bgcolor), var(--toolbar-non-lwt-bgcolor) 45px);
 }
 
 #customization-palette {
   padding: 5px 20px 20px;
 }
 
 #customization-header {
-  font-weight: 500;
+  font-weight: 600;
   font-size: 1.2em;
   margin: 20px 20px 15px;
 }
 
 :root[lwtheme-image] #customization-header {
   text-shadow: 0 0 1em var(--toolbar-non-lwt-bgcolor),
                0 0 1em var(--toolbar-non-lwt-bgcolor),
                0 0 .5em var(--toolbar-non-lwt-bgcolor);
@@ -370,17 +370,17 @@ toolbarpaletteitem[place=toolbar] > tool
   font: menu;
 }
 
 #customization-lwtheme-menu-header,
 #customization-lwtheme-menu-recommended {
   padding: 10px;
   margin-bottom: 5px;
   text-align: center;
-  font-weight: 500;
+  font-weight: 600;
   border-bottom: 1px solid var(--panel-separator-color);
 }
 
 #customization-lwtheme-menu-header,
 #customization-lwtheme-menu-recommended,
 #customization-lwtheme-menu-footer {
   margin-right: -10px;
   margin-left: -10px;
@@ -483,17 +483,17 @@ toolbarpaletteitem[place=toolbar] > tool
 #customization-panelDescription {
   font-size: 1.1em;
   padding: 2px 12px 10px;
   margin: 0;
 }
 
 #customization-panelHeader {
   font-size: 1.3em;
-  font-weight: 500;
+  font-weight: 600;
   padding: 2px 12px;
   margin: 0;
 }
 
 #customization-panelHolder > #widget-overflow-fixed-list {
   padding-top: 10px;
 }
 

--- a/browser/themes/shared/customizableui/panelUI.inc.css
+++ b/browser/themes/shared/customizableui/panelUI.inc.css
@@ -1498,17 +1498,17 @@ toolbarpaletteitem[place="menu-panel"] >
   flex: 1 auto;
   height: 40px; /* fixed item height to prevent flex sizing; height + 2*4px padding */
   padding: 4px;
 }
 
 .panel-header > label {
   flex: auto;
   font-size: 13px;
-  font-weight: 500;
+  font-weight: 600;
   margin: 0;
   text-align: center;
 }
 
 .panel-header > .subviewbutton-back + label {
   /* Add the size of the back button to center properly. */
   margin-inline-end: 32px;
 }