Всеволод Родионов

var FILE = 'menu.less',
    SELECTOR = '.icon-hh:hover::before';

var sheets = document.styleSheets,
    sheet,
    i = 0,
    imax = sheets.length,
    rule;

for (; i < imax; i++) {
    sheet = sheets[i];
    if (sheet.href && sheet.href.indexOf(FILE) !== -1)
        break;

}

if (i === imax)
    throw new Error('File Not Found');

i = 0;
imax = sheet.cssRules.length;

for (; i < imax; i++) {
    rule = sheet.cssRules[i];
    if (rule.selectorText === SELECTOR)
        break;
}

if (i === imax)
    throw new Error('Selector Not Found');

rule.style.background = 'green';