If using the "Classic" workspace,  under the pulldown menu option "Window", you can execute the option "Switch UI Mode".
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Alternatively you can type _UISTYLE into the command line and press <RETURN>.

You can also access this setting via Options>System Options, Display> User Interface Style (from any workspace).
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Note: Regardless of the method (of access used), the change to the (opposite) UI will only take place after you restart ARES Commander.