I know it's possible to change the background color of cells in a table but is it possible to change the size and the color of the border lines ?

No idea about how to change the colour of the border lines.

Changing their width is easy. Once the table is created, put the cursor inside the table. Clic on the icon of the table icon bar that says Cell to select if you want to change the borders of a cell, a line, a column or the table. Then, clic on the icon "Change border of cell" (the one with a blue corner). In Border, select the border you want to change. After that in the status line, at the bottom of the window, you will get a question and there you put the width of the border. For example, 1ln. TeXmacs uses several length units. More information about length units here:

http://www.texmacs.org/tmdoc/devel/format/basics/lengths.en.html

I hope that it's clear…

Javier