Add transparency effects toggle to settings

src/inject/injected.css

@@ -1,6 +1,7 @@
 @import url("https://fonts.googleapis.com/css?family=Rubik:300,400,500,600");
 @import "./injected/popup.css";
 @import "./injected/sidebar-animation.css";
+@import "./injected/theme.css";
 
 :root {
   background: var(--better-main) !important;

src/inject/injected/theme.css

@@ -0,0 +1,11 @@
+html.dark {
+  --background-primary: #232323;
+  --background-secondary: #1a1a1a;
+  --text-primary: white;
+}
+
+html:not(.dark) {
+  --background-primary: #ffffff;
+  --background-secondary: #e5e7eb;
+  --text-primary: black;
+}