My smoked plexi is 3mm so 1/8" and is grey not brown, as to light/dark I would assume mine is light. I cannot imagine 1/4" dark working that well, I would think it is too dark. I didn't have a choice as to light or dark, or grey or brown, they only had one kind of smoked plexi.
The 1/8" is thick enough if you don't plan on leaning or pressing on it. 1/4" is pretty thick stuff, take a look at some and see if you like it.
I simply cut out some black poster board and taped the outside edges to the back of the plexi as a bezel, it looks pretty good, nice and black.
What I would suggest is going down to the store and taking a look at the product if you can and make your own decision. Take a potable TV or LCD display if you can and hold it behind the plexi and see what looks good.