Small bathrooms without windows can be a bummer for renters, however there are tricks you can use to make the room more welcoming. Check out Apartment Therapy's suggestions for turning your apartment bathroom into a lighter, brighter space.
Even in a windowless bathroom, an air plant or shade-loving plant can work if they don't need much light. You may need to occassionally move them around to a window to sneak a little bit of light now and then. Another non light-dependent option is to add fresh flowers.
Usually one over the mirror light isn't going to be enough to illuminate the space. So add sconces, wall lights, or even a hanging light to make things a lot more bright.
If you have room, a piece of furniture like a stool, chair, or small chest can really make your bathroom unique.
If painting's an option, use a bright white or other light color to imbue as much light as possible into your space. Even if you can't paint, add mirrors to reflect whatever light is available in your space.
Use color and pattern through a bold shower curtain or temporary wall paper, to distract from the lack of mirrors.