If set to adw.types.ToolbarStyle.Flat, bottom bars are flat and scrolling content has a subtle undershoot shadow when touching them, same as the .undershoot-bottom style class. This works well for simple content, e.g. adw.status_page.StatusPage or adw.preferences_page.PreferencesPage, where the background at the bottom of the page is uniform. Additionally, windows with sidebars should always use this style.
Undershoot shadow is only present if a bottom bar is actually present and visible. It is also never present if adw.toolbar_view.ToolbarView.extendContentToBottomEdge is set to TRUE.
If set to adw.types.ToolbarStyle.Raised, bottom bars have an opaque background and a persistent shadow, this is suitable for content such as utility panes, where some elements are directly adjacent to the bottom bars, or adw.tab_view.TabView, where each page can have a different background.
adw.types.ToolbarStyle.RaisedBorder is similar to adw.types.ToolbarStyle.Raised, but the shadow is replaced with a more subtle border. This can be useful for applications like image viewers.
See also adw.toolbar_view.ToolbarView.topBarStyle.