border-width: 120px 120px 0 0;
border-color: #4f46e5 transparent transparent transparent;
The CSS Triangle Generator widget allows you to generate CSS triangles in different sizes, directions and colors. To understand how it exactly works check out CSS Triangles: A Tragedy in Five Acts.
You can select your desired shape, choose your own color and adjust its size proportionally or individually. You will get a final output in the right part which you can easily copy.