The fork's colour/wallpaper/kvantum scripts still hard-code
QUICKSHELL_CONFIG_NAME="ii", so they read and write
~/.config/quickshell/ii instead of end4-pC - rewrite them to end4-pC
at build time (verified against the current fork HEAD: 5 scripts match
the anchored pattern, re-runs are no-ops).
Also replace the fork's assets/images/default_wallpaper.png - fed to
switchwall.sh by FirstRunExperience.qml on first login - with the
wallpaper now shipped in skel's ~/Pictures/Wallpapers, so the first
login lands on the japan-street wallpaper instead of the fork's
default.
