New (semi-official) method

[s-hager]

Just found this method posted to reddit here:

I've discovered a way to make fusion darker, its not blackout, but its much better than default. Here are the steps:
Open a folder and go here:
C:\Users\YOUR_USERNAME\AppData\Local\Autodesk\webdeploy\production\bce2902bbfcb27678033cbb9e17a3529631b97a7\Neutron\UI\Base\Resources\UIToolkit
Edit the file named "theme-featurepacks.xml"
On line 7 you'll see this line of code - <Feature Id="ui-themes-enable" Default="False" RequireRestart="False">
Change Default="False" to Default="True"
Restart Fusion360
On the top right, click on the user icon, and go into preferences
There will now be a new option called Theme.
Select the Dark Blue theme
the background all goes white so its hard to see once you select it, but the buttons at the botton right in order are:
Apply - OK - Cancel
Hit Apply, then OK and restart Fusion360

Also changing the environment to Dark Sky helps also... 

What's also funny is that in this folder there seems to be an official demo .gif showing off the dark mode: AppData\Local\Autodesk\webdeploy\production\bce2902bbfcb27678033cbb9e17a3529631b97a7\Neutron\UI\Base\UI\Controls\DarkMode\Windows

So it seems the official dark mode is in development and might not be totally complete in some areas but this still works quite well nonetheless.

[ctsUX]

Working great on Mac! For anyone curious, this makes the default UI match the "River Rubicon" environment background, with River Rubicon then becoming the default background. Thanks for sharing!

Update: you can now just go to Preferences -> Preview Features (all the way at the bottom) -> Enable UI Themes -> then go to General (at the top) -> Theme and select Dark Blue. Enjoy!

[SoCuul]

With this new dark theme, it looks like they've actually built out an entire theming system. If I'm correct about this, that means that having fully custom theme colours should be achievable!