Dark Mode based on Client Local System Theme

[chiragthakur09]

1. This PR is addressing the issue Feature request: Dark mode #1893, A new feature to set Dark mode based on Client local systems theme.

When Client System Theme is Set to Light

When Client System Theme is Set to Light

Dialogs had internal css

Replaced with Primers vars

• Replaced Dialogs internal css with Primer var

[dometto]

@chiragthakur09 the fix for the Dialogs looks good, but the other items on the to-do list in #1938 still seem unresolved in this PR. Maybe you could copy/paste all the Todo items to this PR? (We can't really merge until the todos are all handled, since the official Docker image is based on what we merge into master -- so Docker users would get an incomplete dark mode, with the Todo items unresolved)

[dometto]

I'm going to close this for lack of activity, but the fix for our dialogs was very helpful! I'm pushing it to #1938 and have credited the commit to you, @chiragthakur09. If you want to continue implementing the todo items in #1938 we would be very grateful and we can reopen this PR. Otherwise I'm going to assume that development continues in #1938.