Modifying the alpha channel of a color using CSS

Suppose you have a color defined in a CSS variable (e.g., var(--brand)), and you want a more transparent version of it (for example, 75% opacity). You can achieve this using the color-mix() function in CSS:

background: color-mix(in srgb, var(--brand) 75%, transparent);

Equivalent in SCSS, Less, and Stylus

SCSS:
```
color: rgba($brand, 75%);
```
Less:
```
color: rgba(@brand, 75%);
```
Stylus:
```
color rgba(brand, 75%)
```

Reference

StackOverflow Question: How do I apply opacity to a CSS color variable?