I researched and found a solution. I discovered that it is possible to customize the bar with these CSS properties:
Setting properties:
The scroll bar in general:
::-webkit-scrollbar{width:15px; }
The up / down buttons:
::-webkit-scrollbar-button{background-color:#bfbfbf; height:15px;}
From the background behind the scroll bar:
::-webkit-scrollbar-track-piece {background-color:#e4e4e4;}
Vertical scrollbar:
::-webkit-scrollbar-thumb:vertical{background-color:#bfbfbf; border-top:1px solid #a4a4a4; border-bottom:1px solid #a4a4a4}
Horizontal scrollbar:
::-webkit-scrollbar-thumb:horizontal{background-color:black;}
Bottom (vertical bar) button:
::-webkit-scrollbar-button:vertical:increment {background-image:url("Imagens/seta-baixo.png");}
Top button (from the vertical bar):
::-webkit-scrollbar-button:vertical:decrement {background-image:url("Imagens/seta-cima.png");}
Result: