If you decide to modify LESS source files, remember that switching from one theme to another require only to change theme variable at line 2 in following file.

Source file less/asciidoc-bootstrap.less. 

@import "variables.less";
@theme:     "default";
@themesdir: "../themes/@{theme}";

// Bootstrap 3.2
@import "bootstrap/bootstrap.less";
@import "@{themesdir}/variables.less";

// Font-Awesome 4.2
@import "font-awesome/font-awesome.less";

// Ionicons 1.5
@import "ionicons/ionicons.less";

// Octicons 2.1 with '@octicons-font-path' modified
@import "octicons/octicons.less";

// AsciiDoc 8.6
@import "asciidoc/common.less";
@import "asciidoc/pygments.less";
@import "asciidoc/iconsfont.less";
//@import "asciidoc/admonition-text.less";
@import "asciidoc/admonition-iconsfont.less";