fixed massive performance hit in Chromium v80 (Electron v8) when background color is applied to all elements, the workaround is to use transparent instead.

Author: danielweck

css/dist/ReadiumCSS-after.css

@@ -24,7 +24,7 @@
 /* we don’t need to redeclare bg-color and color for :root since we will inherit and update from day/default mode */
 
 :root:--night-mode *:not(a) {
-  background-color: var(--RS__backgroundColor) !important;
+  background-color: transparent !important;
   color: inherit !important;
   border-color: currentColor !important;
 }

css/dist/cjk-horizontal/ReadiumCSS-after.css

@@ -25,7 +25,7 @@
 /* we don’t need to redeclare bg-color and color for :root since we will inherit and update from day/default mode */
 
 :root:--sepia-mode *:not(a) {
-  background-color: var(--RS__backgroundColor) !important;
+  background-color: transparent !important;
   color: inherit !important;
 }