The accessibility menu is where you can turn on some of the accessibility settings. You can find this menu by clicking the icon which looks like a person on the top bar.

If you do not see the accessibility menu, you can enable it from the Accessibility settings panel:

To access this menu using the keyboard rather than the mouse, press CtrlAltTab to move the keyboard focus to the top bar. The Activities button in the top-left corner will get highlighted — this tells you which item on the top bar is selected. Use the arrow keys on the keyboard to move the selected item to the accessibility menu icon and then press Enter to open it. You can use the up and down arrow keys to select items in the menu. Press Enter to toggle the selected item.