Finish up new sidebar active.

Add rounded corners to sidebar.

src/styles/_variables.scss

@@ -0,0 +1,3 @@
+:root {
+    --sidebar-active: var(--secondary-background);
+}

src/styles/index.scss

@@ -1,3 +1,4 @@
+@import "variables";
 @import "context-menu";
 @import "elements";
 @import "fonts";