Can't install theme

hello im newbie here..
i tried to install some theme for my fresh installed fedora, but it seems not working

did i made some mistake? please help. thanks
pardon my english

Simple really.

Dnf installs .rpm files and you seem to have a compressed .tar.xz file which is not manageable with dnf.
Extract the .tar.xz file and there is probably either a script or a readme file that will tell you how to install it.

Have you seen that kvantum is in the fedora repo?

sudo dnf install kvantum would work. It is just not 1.1.8 however must be a small difference to V1.1.6.

It even has a message in the package system that mentiones 1.1.8 is available.

If you install downloaded tar.xz files, you could get into troubles with dnf in the future, not being able to install the official packages. Be careful.