An attacker could plant a malicious DLL file in your "Downloads" directory through drive-by downloads or social engineering, enabling them to execute arbitrary code on your system when you install an infected theme. This technique, known as DLL planting, allows malware to execute with the privileges of the user installing the theme.
: Users append this keyword to filter for rare, third-party, or premium theme packs that were not part of the standard Microsoft release. What are Windows 7 Themes? intitle index of windows 7 themes exclusive
They are; Nightlight, a theme that gives your computer a Windows 10 nightlight feel, Nighttime, a theme inspired by the night sky, Beta Archive An attacker could plant a malicious DLL file
Although the original Microsoft Personalization Gallery is no longer active, understanding its structure can help locate mirrored copies: What are Windows 7 Themes