to change the text ‘categories’:
.sidebar-widget.widget_product_categories .widget-title { color: #000; }
change the colour code as you want
to change the block / background color:
.widget_product_categories .widget-title { background-color: #FF0; }
to add the border/outline :
.sidebar-widget.widget_product_categories .widget-title { border: 2px solid #000;}
put the above code in 8theme options >> styling >> custom css >> global custom css
change the color code and combine the combination as you want.