The login screen in Windows 10 no longer has that plain-colored background that was present in Windows 8.1. You could say that the new login screen is more “alive” now that it features the default Windows 10 wallpaper as its background, but unfortunately, it isn't customizable through settings.
Although it can still be removed to bring back the single-color, theme-based background, here’s how to do it.
Step 1: Hit Windows Key + R to bring up the Run dialog, then type ‘regedit’ and hit Enter
Step 2: On the left pane in Registry Editor, navigate to HKEY_LOCAL_MACHINE, Software, Policies, Microsoft, Windows, then System
Step 3: Right-click anywhere in the right pane and select New, DWORD (32-bit) Value, then name it DisableLogonBackgroundImage
Step 4: Right-click the newly created value, then click Modify
Step 5: Change the 0 under ‘Value data’ to 1, and click OK
There you have it. You can test if it worked by locking your PC (Windows Key + L), notice that the background is the same color as your theme, which can be changed in Settings, Personalization, Colors.
Let us know in the comments section below whether you'll be sticking with the Windows logo background on your log-in screen or changing it to your theme color.