When Gulp compiles, I get this error:
error evaluating function `darken`: undefined is not a function in file /path_to_theme/bower_components/bootstrap/less/variables.less line no. 36
error evaluating function `darken`: undefined is not a function in file /path_to_theme/bower_components/bootstrap/less/variables.less line no. 36
The variables.less file is the default one, that is:
//
// Variables
// --------------------------------------------------
//== Colors
//
//## Gray and brand colors for use across Bootstrap.
@gray-base: #000;
@gray-darker: lighten(@gray-base, 13.5%); // #222
@gray-dark: lighten(@gray-base, 20%); // #333
@gray: lighten(@gray-base, 33.5%); // #555
@gray-light: lighten(@gray-base, 46.7%); // #777
@gray-lighter: lighten(@gray-base, 93.5%); // #eee
@brand-primary: darken(#428bca, 6.5%); // #337ab7
@brand-success: #5cb85c;
@brand-info: #5bc0de;
@brand-warning: #f0ad4e;
@brand-danger: #d9534f;
//== Scaffolding
//
//## Settings for some of the most global styles.
//** Background color for `<body>`.
@body-bg: #fff;
//** Global text color on `<body>`.
@text-color: @gray-dark;
//** Global textual link color.
@link-color: @brand-primary;
//** Link hover color set via `darken()` function.
@link-hover-color: darken(@link-color, 15%);
//** Link hover decoration.
@link-hover-decoration: underline;
...