Be cautious. Applying a tinting material such as is used on car windows would be difficult due to the curvature of the lens. If you were to use a chemical to tint the glass or plastic lenses, you may damage the glasses and you wouldn't know if the chemical was safe to have near your eyes. May not be worth the risk.
No. Lenses are carefully made to limit uv light - if your base lenses aren't uv treated and they are made darker actually more uv can enter the eye with the dark lenses (pupil opens)!