Bug 1549282 - Remove checkbox-label appearance style for in-content pages to avoid interfering with selection highlight color in dark mode on Linux r=dao

`-moz-appearance: checkbox-label` causes in-content light text to behave like dark text for selection purposes, the primary color is used for the background highlight instead of the alternate. Setting `-moz-appearance: none` for checkbox labels avoids the problem.

Differential Revision: https://phabricator.services.mozilla.com/D29988

--HG--
extra : moz-landing-system : lando

toolkit/themes/linux/global/in-content/common.css

@@ -10,8 +10,12 @@ xul|tab[visuallyselected] {
   margin-bottom: 0;
 }
 
+/* Overriding appearance also avoids incorrect selection background color with
+   light text. */
 xul|button > xul|*.button-box,
-xul|menulist > xul|*.menulist-label-box {
+xul|menulist > xul|*.menulist-label-box,
+xul|*.radio-label-box,
+xul|*.checkbox-label-box {
   -moz-appearance: none;
 }
 
@@ -39,10 +43,6 @@ xul|*.radio-check[selected] {
   fill: -moz-fieldText;
 }
 
-xul|*.radio-label-box {
-  -moz-appearance: none;
-}
-
 xul|menulist:-moz-focusring > xul|*.menulist-label-box {
   outline: none;
 }