materia-gtk-theme: Update to 20190831

Notable changes:

Colors

    Darken primary color in light variants
    Lighten primary color in dark variant
    Lighten separator color in dark variant
    Darken dark color scheme slightly
    Change inactive window's title bar background-color
    Update error/warning/success/visited colors to meet accessibility standards

Styles

    Change focus style from outline to overlay
    Change all drop style to much hover style
    Change disabled scale style
    Change hover style of combo buttons
    Change combo entry style
    Change osd toolbar style like YouTube's floating toolbar
    Update typography in gnome-shell
    Update style and logic of shadows to much the latest Material Design guidelines
    Update stack sidebar style
    Update info bar style
    Adjust text button sizing
    Use primary color for dialog buttons
    Remove a separator from LightDM's dialog
    Normalize tab bar height in gnome-terminal & tilix

Others

    Make transition a little faster
    Sync GTK2 entry/combobox/spinbutton styles with GTK3


(nia)