put a class on your table like class="mytable"
then in the css for your skin add
table.mytable td, table.mytable th { border:solid thin black; }
obviously you can use different width colors etc by using different css rules.
After editing the css for your skin, go to Administration > Advanced Tools > Design Tools, Cache Tool and click th ebutton to reset the skin guid, tha will force all browsers to load the latest css.
Hope that helps,
Joe