Steps

1. Find the property you want to use.

XWiki Standard has its LESS variables declared in a few files, mostly:

• variables.less from the bootstrap module: pretty much everything to make the Bootstrap 3 design system work.
• variables.less from the flamingo module: XWiki's special variables and overrides for default values
• variablesInit.vm from the flamingo module:
• $xsVariablesValues in variablelist.vm 
• The flamingo Color Themes, those variables can be viewed and edited directly in the Administrator UI but they do not cover everything in the skin.

2. Use this syntax in a LESS stylesheet

.box.box-custom {
  color: @brand-color;
}

FAQ

Can I use a LESS variable in a CSS stylesheet?

Technically this is impossible. Use a CSS property instead. In the rare case where you NEED a CSS stylesheet and cannot use CSS properties, rely instead on the old Colortheme variables.