Add a way to change theme (dark/light) from command line

I love the dark/light theme support in Discord! But unfortunately I use a lot of applications, and changing the theme on all of them 2-3 times per day is a lot of time wasted.

On Windows/Mac, you could simply follow the system wide dark theme setting (I'm a Linux user, so I'm not sure if you've already implemented that?)

However, Linux doesn't yet have such a system wide dark theme standard, although that will hopefully change soon. (I really enjoyed this blog post on the topic by one of the co-founders of Elementary OS: https://blog.elementary.io/the-need-for-a-freedesktop-dark-style-preference/)

For now though, it would be amazing if you could simply add a way to control the theme from the command line, so me and my fellow Linux users could at least automate switching between light and dark themes.

For example:

    discord --theme dark

Would switch to the dark theme in the existing Discord instance.