Fixes#1348
Tried to get the unread messages indicator to line up properly with the
text and icons in a room list item.
Also tried to reuse styles where appropriate, specifically
`.list-container` and `.list-toggle`.
This theme takes inspiration from https://discordapp.com/ and builds on
top of custom-properties, making the design more familiar to some users.
In order to change the theme, add the 'theme-dark' class on #conversejs,
you can do it at any point during the lifetime of Converse, either
directly in your HTML or by changing the DOM at runtime.
