Hi Maserati,
We do not recommend making changes to the smart.default.css file because this could lead to unwanted behavior of the components.
When you want to change styling of a component its best to do it through its CSS variables or selector.
For example if you want to set up the font-size in SmartGanttChart:

    smart-gantt-chart {
        width: 100%;
        height: auto;
        --smart-font-size: 25px;

More info about the CSS variable and selectors you can find the documentation for each component.
For the GanttChart:
Please, do not hesitate to contact us if you have any additional questions.
Best regards,
Yavor Dashev
Smart UI Team