Hi tuillio0106,
I have code snippet for you to showcase you how set a CSS variable in the SmartTextBox component.

smart-text-box {
    --smart-font-size: 25px;

You have to set the CSS variable itself and to ‘force’ set the property with the font-size attribute.
Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team