I want to make my own theme, but?

After I get done editing a wordpress theme someone else made, it never looks right on anyone else’s computer. I’m tired of messing with themes for hours to get them to look how I want, so I just want to make my own. The problem is, I’m kinda clueless. I made a theme (well, I stole PHP files from someone elses and then just tried to use my own style.css lol.) and whenever I tried to switch to it was like, your attempt failed. I have no idea why, but I think it’s because I have no idea what to put for theme’s uri. Help?!

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