Check out example codes for "css opacity background color". It will help you in understanding the concepts better.

Code Example 1

background: rgba(255, 255, 255, 0.25);

Code Example 2

rgba(51, 170, 51, .1)    /*  10% opaque green */ 
rgba(51, 170, 51, .4)    /*  40% opaque green */ 
rgba(51, 170, 51, .7)    /*  70% opaque green */ 
rgba(51, 170, 51,  1)    /* full opaque green */

Code Example 3

h1 {background-color:rgba(255,0,0,0.3);}

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