add modifications necessary for Rocket.Chat 6.0.0

[paulchen]

A few minor notifications that are necessary to make the dark mode work with Rocket.Chat 6.0.0.

Some additional tweaks might be necessary in order to make it more beautiful, but in general it works as far as I can tell.

[chotaire]

Thanks for your work @paulchen please check out https://github.com/pbaity/rocketchat-dark-mode/issues/213