A user on reddit recently asked how to modify the Breeze Alt+Tab theme. Unfortunately it seems to be tied to the Look and Feel as of Plasma 5.14.

Introduction to KWin TabBox skins

The tabbox skins for the Alt+Tab are located at /usr/share/kwin/tabbox/ for the preinstalled skins, and ~/.local/share/kwin/tabbox/ for the downloaded skins. However, you’ll notice that the “Breeze” tabbox skin isn’t there. This is because it’s packaged in the Breeze “Look and Feel”.

/usr/share/plasma/look-and-feel/org.kde.breeze.desktop/contents/

I have no idea what the “desktop” switcher is. It’s not the “Desktop Switch On-Screen Display”.
desktopswitcher/DesktopSwitcher.qml

The Alt+Tab skin is probably the:
windowswitcher/WindowSwitcher.qml

Creating a new Look and Feel

Here’s how create your own look and feel based off Breeze.

mkdir -p ~/.local/share/plasma/look-and-feel/
cp -r /usr/share/plasma/look-and-feel/org.kde.breeze.desktop ~/.local/share/plasma/look-and-feel/
mv ~/.local/share/plasma/look-and-feel/org.kde.breeze.desktop ~/.local/share/plasma/look-and-feel/SystemdPlusLinux
kwriteconfig5 --file=$HOME/.local/share/plasma/look-and-feel/SystemdPlusLinux/metadata.desktop --group="Desktop Entry" --key="X-KDE-PluginInfo-Name" "SystemdPlusLinux"
kwriteconfig5 --file=$HOME/.local/share/plasma/look-and-feel/SystemdPlusLinux/metadata.desktop --group="Desktop Entry" --key="Name" "Breeze (SystemdPlusLinux)"
rm ~/.local/share/plasma/look-and-feel/SystemdPlusLinux/metadata.json

Note, X-KDE-PluginInfo-Name must be the same as the folder name.

Note, you might want to manually remove all the translated Name[sl]=Sapica in the metadata.desktop if you’re not using the en_US locale/translations, as otherwise it’ll use a translated version of “Breeze” in the System Settings instead of what we just set it as (Breeze (SystemdPlusLinux)).

Editing the Look and Feel

Then play around with the WindowSwitcher.qml file.

To test your changes, you’ll probably need to run kwin_x11 --replace to restart KWin. You might be able to get away with switching to another Look and Feel, then back to yours, but I believe it might cache the theme, which is why restarting kwin might be required.

You can also play around with the Volume OSD.

Look and Feel Explorer

There’s also “Look and Feel Explorer” app in the plasma-sdk package, but it’s probably better to learn how to edit the raw files since all that app does is “copy a look and feel”, and edit it’s metadata.desktop file.