Ah! yes I now get it!... I guess its different in Textmate2. All you need to do is to goto the bundle editor and change the value of background in settings.
For example if the value of background is #141414 then append the hex value of the desired transparency. For example you want the opacity to be 62.8% then you can change the value to be #141414A1.
0xA1 = 161
161/256 = 62.8%