My youtube dark theme is turned off every time the browser restarts.
Please try adjusting your per site shields settings. When you’re on YouTube, click Brave icon at top right. Under Cookies control change the setting to Allow all cookies and see if it’s can help.
I think that is normal behaviour of the youtube dark mode in any browser.
No it’s not. Since switching to Brave, this is the first time I’ve had to think about turning it back on, let alone every time. All the while I was using Chrome and then Waterfox, I only had to switch it on once, then I could forget about it forever.