You can achieve this by using rgba() function for your div background
rgba(R, G, B, A)
R (red), G (green), and B (blue) can be either <integer>s or <percentage>s, where the number 255 corresponds to 100%. A (alpha) can be a <number> between 0 and 1, or a <percentage>, where the number 1 corresponds to 100% (full opacity).
RGBa example
background: rgba(51, 170, 51, .1) /* 10% opaque green */
background: rgba(51, 170, 51, .4) /* 40% opaque green */
background: rgba(51, 170, 51, .7) /* 70% opaque green */
background: rgba(51, 170, 51, 1) /* full opaque green */