You can easily add a opacity level to your elements by using RGBA (Red, Green, Blue, Alpha) instead of your regular HEX or RGB color scheme.
The alpha channel is the amount of opacity.
If you would like a font in complete black and then use 75% opacity the code would be:
This will let your color appear black, but with the opacity set to 75%.
A: 0.00 - 1.00 (0.00 is 0% and 1.00 is 100% - 25% would be 0.25)
You can use the RGBA value on all the elements where you would usually use the HEX value.
You can use an online colorpicker if you don't already know the RGB value of your hex, such as Colorpicker.com. When using Colorpicker.com and you already know the HEX value, you can easily find the RGB value by entering the hex value after the link.
Such as: http://www.colorpicker.com/4CB8D9