Uncheck the "Highlight the squares of the last moves made" option. (SETTINGS - STYLE)
how to remove the yellow border

kewuchiha,
There is an option to remove the last highlighted move in live chess.
Access live chess, go to your settings either found in the upper right hand corner of the screen or directly below the main chat box, in the new box that opens go to the "style" tab, then uncheck the "highlight last move" box.
However, please note that we have confirmed a bug with this feature and are working hard on getting it fixed. If you disable this feature and then try to make a move by clicking your piece and then clicking its destination square rather than clicking and dragging, you will not be able to move.
So, just a heads up before you go ahead and disable highlighting :-)

ok Kansha
how to remove the yellow border