1) Open your WP themes “style.css”.
and add large number of version.
Ucan stop update Theme to add one character !
2) Open your WP themes “style.css”.
and The LAST Line write like this.
/****************my origenal custom*******************/
write some css decolations under there.
If your theme inform an update.
You copy & paste incrude /***my origenal custom***/ below all css to new updated theme’s css.