For people with Windows 8 or XP, I'm sorry to say that I don't know how to do it with them, it may be the same but this is towards the Windows 7 people.
BEFORE ANYTHING, CREATE A RESTORE POINT if anything goes wrong, go in safe mode and the restore point will save you.
-Search for "Windows 7 theme patcher" in google and click something that seems legit to you
-Run the patcher
-Restart your computer
-Go on deviantart and go in Customization/Skins & Themes/Shell Themes
-Find the one of your choosing, if it does not include a readme then go to wherever your windows folder is the go to Resources/Themes
-Double Click the folder ONCE and if there is an en-us folder, a shell folder, and a Theme file, if there is then drag the folder with those files and folders into the destination I told you before.
-Go to personalize then click the theme that you've just installed
Explorer should refresh and it should look like the theme you wanted!
And please no comments like "Oh I already knew how to do this and it's easy this was a useless post leleellel"
This is for the people who
DON'T KNOW, not the people who do know.