ok if it was downloded from a MSoft site : Go to that website
again an download it again : or do you see it . When you
open "Personalized" as unselected. click to add it
As your - Desktop win's 7 backround" Saved and ready
Restart at least twice to check. if Stay on : Now if the
theme isn't available. You can download
a new one from this site below : if need help on how
to install a customized theme : Watch the video
once opened the link with "Win's 7 themes online" click "View All"
when scrolling and find "one wins 7 theme or background wallpaper"
click it to View more < > with windows 7 logos : once you find one
that, you like it > Righ click and from the options > select the
< Save as image > click [Save] button : Next open
"File Explorer" Find the theme file in the - Downloads folder and click
< open > it may appears the option "Save it as desktop backround"
Link : https://www.google.com/search