Yes, I would consider this to be a simple implementation.
Today, you can switch from Left Nav/Header Drop-down by changing a global site-setting in real-time. This indicates there is switch (if statement or equivalent) that evaluates a global site setting and sets the UI. Since this switch is making this decision from a global site setting, adding a user defined site setting is building out the “or” functionality in the switch.
IF (global site setting or user site setting) then … else…
If flipping the switch between Left Nav/Header Drop-down required a recompile for the change, then different story…