fix: proton mail sidebar color

Author: juliamertz

styles/proton/rose-pine.user.less

@@ -157,6 +157,7 @@
       --border-norm: @overlay;
       --border-weak: @overlay;
       --background-norm: @base;
+      --background-lowered: @surface;
       --background-weak: fadeout(@highlightHigh, 80%);
       --background-strong: @highlightHigh;
       --background-invert: @text;