KDE: Complete fix for padding and tiny min/max/restore/close buttons for GTK programs.

gtk.css (gtk-3.0)

/* Edit .config/gtk-3.0/gtk.css and add: */

headerbar, headerbar arrow, headerbar box, headerbar button, headerbar contents, headerbar entry, headerbar image, headerbar menubutton, headerbar slider, headerbar splitbutton, headerbar trough {
    margin: 0;
    min-height: 0;
}

headerbar {
    border-top: 0;
    padding-right: 0;
}

button.minimize {
    min-height: 25px;
    min-width: 25px;
    border: 0px;
    color: transparent;
    background-size: 25px;
    background-image: url('assets/minimize-normal.svg');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center;
}

button.minimize:hover { background-image: url('assets/minimize-hover.svg') }

button.minimize:active { background-image: url('assets/minimize-active.svg') }

button.maximize {
    min-height: 25px;
    min-width: 25px;
    border: 0px;
    color: transparent;
    background-size: 25px;
    background-image: url('assets/maximize-normal.svg');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center;
}

window.maximized button.maximize { background-image: url('assets/maximized-normal.svg'); }

button.maximize:hover { background-image: url('assets/maximize-hover.svg') }

window.maximized button.maximize:hover { background-image: url('assets/maximized-hover.svg') }

button.maximize:active { background-image: url('assets/maximize-active.svg') }

window.maximized button.maximize:active { background-image: url('assets/maximized-active.svg') }

button.close {
    min-height: 25px;
    min-width: 25px;
    border: 0px;
    color: transparent;
    background-size: 25px;
    background-image: url('assets/close-normal.svg');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center;
}

button.close:hover { background-image: url('assets/close-hover.svg') }

button.close:active { background-image: url('assets/close-active.svg') }

gtk.css (gtk-4.0)

/* Edit .config/gtk-4.0/gtk.css and add: */

headerbar entry,
headerbar spinbutton,
headerbar button,
headerbar separator {
    margin-top: 0px;
    margin-bottom: 0px;
}

headerbar button { min-height: 24px;}

headerbar {
    min-height: 24px;
    padding-right: 0px;
    margin: 0px;
    border-top: 0px;
}

button.minimize {
    border: 0px;
    padding: 0px;
    min-height: 40px;
    min-width: 40px;
    color: transparent;
    background-size: 25px;
    background-image: url('../gtk-3.0/assets/minimize-normal.svg');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: left;
}

button.minimize:hover { background-image: url('../gtk-3.0/assets/minimize-hover.svg') }

button.minimize:active { background-image: url('../gtk-3.0/assets/minimize-active.svg') }

button.maximize {
    border: 0px;
    padding: 0px;
    min-height: 40px;
    min-width: 40px;
    color: transparent;
    background-size: 25px;
    background-image: url('../gtk-3.0/assets/maximize-normal.svg');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: left;
}

window.maximized button.maximize { background-image: url('../gtk-3.0/assets/maximized-normal.svg'); }

button.maximize:hover { background-image: url('../gtk-3.0/assets/maximize-hover.svg') }

window.maximized button.maximize:hover { background-image: url('../gtk-3.0/assets/maximized-hover.svg') }

button.maximize:active { background-image: url('../gtk-3.0/assets/maximize-active.svg') }

window.maximized button.maximize:active { background-image: url('../gtk-3.0/assets/maximized-active.svg') }

button.close {
    border: 0px;
    padding: 0px;
    min-height: 40px;
    min-width: 40px;
    color: transparent;
    background-size: 25px;
    background-image: url('../gtk-3.0/assets/close-normal.svg');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: left;
}

button.close:hover { background-image: url('../gtk-3.0/assets/close-hover.svg') }

button.close:active { background-image: url('../gtk-3.0/assets/close-active.svg') }

userChrome.css (Firefox)

.titlebar-buttonbox {
    appearance: none !important;
    align-items: stretch !important;
}
.titlebar-button {
    border-radius: 0px !important;
    padding: 6px 8px !important;
}

userChrome.css (Thunderbird)

#unifiedToolbar,
#unifiedToolbarContent {
    padding-block: 0px !important
}

.titlebar-buttonbox-container {
    margin-inline-end: 0px !important;
}