Welcome to our Support Forum and thanks for writing in.
The best way to find the CSS or style applied to any element is to use the browser’s element inspector. The following screenshot (click the image to enlarge) shows what selector and declaration you need to override. Put your custom CSS in Theme Options > CSS. That is the area to override theme styles.
You can find the table styles the same way as I’ve showed you. Use the browser’s element inspector. Add a table HTML in the content. If you’re using Cornerstone, use the Raw Content element. You then view the live page and use the browser’s element inspector to view the styles applied to the table.
Regretfully, that’s the only way you can find the styles because the stylesheet of our themes is minified for optimization purposes so it won’t make sense, reading them.
Please also note that we can’t help you with custom CSS coding including answering custom CSS questions. That is outside the scope of our theme support.