fix broken headers' color in ConfigTab

Author: daiyam

browser/main/modals/PreferencesModal/ConfigTab.styl

@@ -182,10 +182,12 @@ apply-theme(theme)
       color get-theme-var(theme, 'text-color')
 
     .group-header
+    .group-header--sub
       color get-theme-var(theme, 'text-color')
       border-color get-theme-var(theme, 'borderColor')
 
     .group-header2
+    .group-header2--sub
       color get-theme-var(theme, 'text-color')
 
     .group-section-control-input