You have to override variable value in your custom theme `_theme.less` file as below:

    @button__color: red;
    @button__background: yellow;

If you did,t configure compilation then refer this [How to configure css compilation?][1]


  [1]: http://magento.stackexchange.com/questions/95420/how-to-update-customize-override-magento-2-css-styles/143973#143973

    - php bin/magento cache:clean