Button Opacity

I have light transparent buttons on my site that upon hover change to the theme color which works great. The only thing I wanted to do add is a black opacity behind the button so the button text stands out more when it is light transparent. I noticed that the light transparent buttons on the fullscreen slider buttons do this which is the look I’m trying to acheive. (See attached images) How do I modify the code below to add the opacity? Thanks!

.clear-button a.avia-button {
background: transparent !important;
border: 3px solid white !important;
}

.clear-button a.avia-button:hover {
   color: white !important;
    border-color: #2c5b9c !important;
     background: #2c5b9c !important;
}

.clear-button a.avia-button { color: white; }