thumbnail caption alignment

Firstly change to using a child theme.
Creating a child theme http://codex.ww.wp.xz.cn/Child_Themes
A child theme will make it much safer to make these css changes.

Then you want to change your image tiles so that rather than using a “margin-botton: 20px;” as part of set the row height, use height instead, so that in your child theme’s style.css

.wp-caption {
margin-bottom: 0px;
height: 200px;
}

If you want to be safe against a really long caption, then use “min-height” instead.