I've had a couple of people using the Thesis theme ask about the same layout issues over the past couple of days. This is specifically related to spacing of checkbox and multiple choice inputs.
The Thesis layout.css file sets a blanket width of 45% for all inputs (somewhere around line 221 if you're interested) which causes the layout problems for these fields.
Also, there is a blanket border style for all inputs (including buttons) which can get annoying if you're trying to use an image button for your form. You can copy the snippet below and paste it at the bottom of your custom.css file and it should override these default Thesis styles.
ul.gfield_radio li input, ul.gfield_checkbox li input {width:5%!important}
.gform_footer input[type=image] {border:none!important; border-color:none!important; background-color:none!important}