Custom defined css classes do not work if I use editor.setStyle(css_text);

I need to set styles using the data I receive from the database asynchronously. But then the css I composed manually gets removed.

const cssComposer = editor.CssComposer;
var sm = editor.SelectorManager;
var sel1 = sm.add('simple-div');
var rule = cssComposer.add([sel1]);
// Update the style
rule.set('style', {
  width: '100%',
  height:'auto',
  padding:'15px',
  float: 'left',
});

the above class .simple-div doesn't exist If I use setStyle()