Accessing software Built-in Theme Classes

Hello, Is there a way to edit a class already built-in in the software?
I want to edit the width of “border-start” & “border-end” so I make the outline of my table thick.
I tried to do a new class and replace it in the theme but I could not get it work.
Else, Can you provide the content of the mentioned classes? I will copy them to a new class of mine and edit it.

What do you mean by “classes?” There are no classes in Manager. Please use terminology from the program.

I mean an HTML Class attribute.

<td class="text-{{ column.align }} border-start1 {% if forloop.last == true %} border-end{% endif %}"

To begin with, themes are written in Liquid, not HTML. See Change the look of forms with themes | Manager.