You can use the KDE color scheme with any theme, as long as they are not kvantum themes. For consistency is better to install Materia-Dark for KDE (or any of the QT versions of the GTK variants) to fit with gtk theme.
Next you have to copy cyberpunk-neon.colors to ~/.local/share/color-schemes
cp cyberpunk-neon.colors ~/.local/share/color-schemes
Now change the color scheme in Settings -> Appearance -> Color. Don't forget to also change Workspace Theme to Breeze if you want the system to follow the Color Scheme.
I have not make a Welcome Screen for KDE, but I recommend using Charlie by l4k1. The text you need to edit is at line 211 in ~/.local/share/plasma/look-and-feel/org.kde.Charlie.desktop/contents/splash/Splash.qml