I tried to change the theme and other related things on system settings but even if i already installed the theme and hit apply but it will not change anything. I tried to uninstall and reinstall it but it is still the same. I tried to go to root and copy the downloded theme from tmp to plasma desktoptheme but it cannot be pasted in there. it will not let me do it. I do not want to download it from the website. Is it a bug or?

Which theme is it? How old is it?
My guess is that is has become outdated, or is buggy, broken, and/or the person who created it no longer maintains it.

If it is an old KDE 4 theme, it might work if you manually download it, and use the Install from File option