So I have a ColumnChart and one of the built-in functionality is that you can hover over an item (so called category) in the legend of the chart and you get some highlight-borde
In the past I used some CSS hacks to change some properties of the generated SVG (path, rect, etc...) you can play around with advanced selectors and maybe you'll be able to achieve you what you want. I created a very quick and buggy example, but maybe it will point you on the right direction.
For example:
div.google_chart svg g g g g rect {
stroke-width:0px; fill:red;
}
Hope it helps.