If you don't already know, Edge Insider does have a dark mode, it's just hidden behind a flag. The update notes suggest it'll be coming soon, but you can get it right now by following these steps:
In Edge Insider's Omnibox (Address Box), go to the address "edge://flags" (Without the quotes) to visit Edge's Flag page. This flag page allows you to enable and disable certain features of Edge, similar to Windows 10's Registry Editor (A.K.A. "regedit")
After the flags page finishes loading, in the top of the page you will notice a search bar. In this search bar, type "#edge-follow-os-theme" (Without the quotes). Only one flag should come up, the "Microsoft Edge theme" flag.
On the right, you will also notice that the flag is selected to "Default". Click on this selection, and a dropdown window will pop up. Select "Enabled" (Without the quotes) from the dropdown window.
Edge will now follow whatever theme is present in the Windows 10 settings. If you have light mode enabled on Windows 10 (Default), then light mode will be present in Edge. To enable dark mode on Windows 10, follow these steps:
Open the Windows 10 settings app by using the keystroke WIN + I or any other method you wish.
Once in this app, select the "Personalization" (Without the quotes) section.
On the right bar, select "Colors" (Without the quotes).
Scroll down, towards the end of the page, until you find a tab that says "Choose your default app mode". Select this option to "Dark" from the default "White" option.
After following these directions, relaunch Microsoft Edge Insider. You will notice that the entire browser has been put into Dark mode, similar to how it is in normal Edge.
That's all! Thanks for reading this guide. If you found it helpful, share it around!