XF 2.1 modify color variables

[Tealk]

Hello

I created a color:
@xf-blue; #005AFF

now i want to use this color for a hover effect but with transparency.
The color code would then be #005AFF25 but I can't figure out how to add this to the variable.

    border-left:7px solid @xf-blue;

    &:hover {
        background-color: @xf-blue -> +25;
    }