Hello Jorge,
I investigated this further and unfortunately it seems that there is no easy way of centering the text inside a tooltip.
The Charts in OutSystems actually use a JavaScript library called Highcharts (http://www.highcharts.com/). This library renders the charts on the screen using SVG. This means that the HTML that you are defining for the tooltip is converted to SVG (even when you define useHtml to yes, which is strange).
I used JSFiddle to play around with the CSS and HTML and even SVG attributes without success: http://jsfiddle.net/ehrqxh41/1/
The Highcharts documentation includes some advanced examples of formatting the tooltip using tables but I wasn't able to obtain the same results: http://api.highcharts.com/highcharts#tooltip
Finally, on the official support forums of Highcharts there is one question on how to center the text in tooltips but the answer doesn't help: http://forum.highcharts.com/post64399.html#p64399