CSS

Satty ships with minimal builtin CSS which can be overridden by $XDG_CONFIG_HOME/satty/overrides.css. Adwaita defaults for headerbar (@headerbar_fg_color and @headerbar_bg_color) which Satty uses ^0.21.0 may lack transparency, here's an override example:

.outer_box,
.toolbar {
    color: #000000;
    background-color: #ddddddaa;
}

You can discover styleable elements by using the GTK inspector with env variable GTK_DEBUG=interactive. Also, see wiki for more examples.