feat: Have theme follow system pref

app/components/DropdownMenu/DropdownMenuItem.js

@@ -1,14 +1,22 @@
 // @flow
 import * as React from 'react';
+import { CheckmarkIcon } from 'outline-icons';
 import styled from 'styled-components';
 
 type Props = {
   onClick?: (SyntheticEvent<>) => void | Promise<void>,
   children?: React.Node,
+  selected?: boolean,
   disabled?: boolean,
 };
 
-const DropdownMenuItem = ({ onClick, children, disabled, ...rest }: Props) => {
+const DropdownMenuItem = ({
+  onClick,
+  children,
+  selected,
+  disabled,
+  ...rest
+}: Props) => {
   return (
     <MenuItem
       onClick={disabled ? undefined : onClick}
@@ -17,6 +25,9 @@ const DropdownMenuItem = ({ onClick, children, disabled, ...rest }: Props) => {
       tabIndex="-1"
       {...rest}
     >
+      {selected !== undefined && (
+        <CheckmarkIcon color={selected === false ? 'transparent' : undefined} />
+      )}
       {children}
     </MenuItem>
   );
@@ -26,6 +37,7 @@ const MenuItem = styled.a`
   display: flex;
   margin: 0;
   padding: 6px 12px;
+  width: 100%;
   height: 32px;
 
   color: ${props =>