fix(buttons): use theme/color toolbar colors for buttons (#18191)

- Updates the iOS buttons in a toolbar to use the proper global theming variables
- Updates the iOS segment to use the correct background variable when checked
- Updates the iOS back button and menu button to use the proper color in a toolbar
- Updates the iOS buttons in a toolbar w/ color to use the proper contrast colors (background, borders, text, hover, focus), mostly solid and outline buttons were affected
- Updates the CSS that applies the global toolbar variables so that it won't affect toolbars w/ a color

fixes #18184, fixes #17840

core/src/components/back-button/back-button.scss

@@ -148,3 +148,10 @@ ion-icon {
 
   pointer-events: none;
 }
+
+// Back Button in Toolbar: Global Theming
+// --------------------------------------------------
+
+:host-context(ion-toolbar:not(.ion-color)):not(.ion-color) {
+  color: #{var(--ion-toolbar-color, var(--color))};
+}