Windows 10 themes get sync between multiple devices, whenever you use your Microsoft account to login into multiple Windows 10 PCs. Sometimes it looks boring to see the same theme on every device. Setting up the different themes on different devices is good idea to make you feel differentiated while using those devices.
If you are willing to use same Microsoft account on all of your Windows 10 PCs then you can stop syncing of Windows 10 themes by following below given process. The alternative way is to create local user accounts separately for each device and get rid of syncing themes.
Disable Windows 10 Themes Syncing Across Multiple Devices
Launch the Settings app on your Windows 10 and click on Accounts.
Click on “Sync your settings” option appearing on the left pane of Settings window.
Now scroll down the screen until you find out the toggle option of “Theme” under “Individual sync settings”. Click on the theme option to turn off the syncing.
Now you can use separate theme on every Windows 10 PC. If you want to turn on the syncing of themes on a later stage, you can enable this option anytime.