how to modify the bottom purple line?

[null1024-ws]

it color, its shape (not a series of dot)

[r00bit1984]

You can do that in the main.css in /assets/css/main.css, under footer.
For example, this is my config, a dashed line:

/* Footer */
footer {
  display: flex;
  align-items: center;
  border-top: 3px dashed var(--bordercl);
  padding: 1rem 0rem;
  margin-top: 2rem;
}

The border color can be set at the very top of the main.css. Link to the file