Hi tullio0106,
First I would suggest you take a look at the CSS API for the input if you need to change the styles of the component(it also has for the texbox element):
And it will be better to explain your use case with a bit more detail about it in order to give a definite solution about it.
However if you want select for styling a certain element inside of SmartInput for example you can do so like this:
//in your HTML file:
<smart-input class="input-class" placeholder="Empty"></smart-input>
//in your CSS file:

 .input-class input{
    background-color: red;

Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team