I guess it just comes down to preference. I assume the “flat” is about this:
I like how the dark theme is now with the grey color scheme they currently have, it reminds me of discord’s dark theme. But some people will be a bit more picky and would like to be able to change the theme colors to their liking just how we can change script editor colors so hopefully that will become an option soon in the future.
Overall good job studio team, I am loving this update! 
Thank You Kanye, Very Cool!
I believe Dark Theme is out!
This is true
The only nitpick I have is like how @Dev_Ryan said, shades currently don’t have enough contrast.
Being able to change UI colors ourselves would be magical though.
You may want to update this post saying that it’s released so people stop messing with FastFlags
Use World Space seems to still have a few issues with the icon 
If I were you I’d probably post that on this Dark Mode issues thread as Roblox staff are more likely to catch it there than in a reply to a community tutorial.
When I go into Studio Mod Manager, it doesn’t show “Edit FVaribles”. Instead, it shows “Edit Fast Flags”. Also, when I click on that and search “StudioThemeSwitchingEnabled”, it says that nothing was found.
This tutorial is over 3 years old – dark theme is no longer hidden behind an FFlag (and hasn’t been for years). Just go into your settings!