@Ismael Martinez The dark theme (dark mode) is available now in the current Dev channel build (Version 188.8.131.52 (Official build) dev (64-bit)). I don't know this for a fact, but I assume that the dark theme is also available in current Canary builds.
By default, Edge Chromium follows the mode used in Windows 10. If Windows 10 is set to light mode, then Edge Chromium is set to light mode. If Windows 10 is set to dark mode, then Edge Chromium is set to dark mode.
I have Windows 10 set to dark mode (Settings, Personalization, Colors), and Edge Chromium (Dev 184.108.40.206) follows that setting, using dark mode by default. This is what Edge Chromium in dark mode looks like on my screen:
I hope that I understood your question, and that this helps.