Hi ON. II'm Greg, awarded MVP for eleven years, Volunteer Moderator, and Independent Advisor here to help you until this is resolved.
You can reset the theme by setting a new Theme from Settings > Personalization > Themes or from the many spectacular free Themes you can download from the Store to that Themes panel to choose from: https://www.microsoft.com/en-us/store/collectio...
To get color back on the task bar, start menu and window frames after Version 1903 turned the Light theme dull grey follow this guide:
https://answers.microsoft.com/en-us/windows/for...
To get the spectacular scenic backgrounds back on the Lock screen when the PC first starts up, go into Settings > Personalization > Lock Screen and set the drop down menu to Windows Spotlight.
If you want the Fun Facts you can hover the mouse over on the Lock Screen or click to go to info pages about the scenes, change dropdown temporarilty to Picture, tick the box to Get fun facts, then change it back to Windows Spotlight.
More about personalizing by customizing appearance here:
https://www.windowscentral.com/how-customize-lo...
https://www.howtogeek.com/225893/how-to-persona...
To save a Windows 10 custom Theme so you can reapply it when Updates or Version mess it up, see here:
https://www.tenforums.com/tutorials/41564-save-...
I hope this helps. Feel free to ask back any questions and keep me posted. If you will wait to choose if I solved your problem, I will keep working with you until it's resolved.
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