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?

- php bin/magento cache:clean

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?

- php bin/magento cache:clean