You can do most of the customization in the default KDE settings. It's fairly easy.

there are a few applications out there, don't know their names. Just google GTK4 theming. Look into youtube aswell.

Terminal is not needed here, no

A lot of people upload their dotfiles when it comes to WMs. Just look up dotfiles on github or usually github dotfiles are posted on unixporn, here's mine for example: https://github.com/Maniacsan/dotfiles