Ub megamenu bug / fails to load style

Dear people,

I have installed crafts and the recommended three extensions. Unfortunatly, the styles won`t show on the page, it`s pure text:
http://dekozeit.quintact.org/
It seems that the template won`t work correctly with the Ub megamenu. Undefined fontsize may be the problem??
How can I get rid of the error?
Thanks for any help,
ina

This is the error message:
Compilation from source:
/kunden/520118_14469/webseiten/dekozeit.com/vendor/magento/theme-frontend-blank/web/css/styles-m.less

Compilation from source:
frontend/Ubertheme/crafts_default/de_DE/css/styles-m.less
variable @font-size-base is undefined in file
/kunden/520118_14469/webseiten/dekozeit.com/var/view_preprocessed/css/frontend/Ubertheme/crafts_default/de_DE/Ubertheme_UbMegaMenu/css/source/module/_horizontal.less
in _horizontal.less on line 361, column 30
359|                 margin-bottom: 0;
360|                 &-title {
361|                   font-size: @font-size-base;
362|                   float: left;
363|                   font-weight: @font-weight__bold;
364|                   padding: 0;