Hi Chris,
You mean how to override theme for a page? If it's what you want, open template style manager that you want to override, in the Theme setting panel, assign a theme for the template style. The assigned theme will override the default theme for pages that assigned to the template style.
To customize a theme, open theme folder you want to customize: templates\t3_bs3_blank\less\themes. Add your custom style in the template.less
When it's done please go to template style manager then compile LESS to CSS.
For more detail, please check it out at:
http://t3-framework.org/documentation/bs3-configuration#theme-setting
http://t3-framework.org/documentation/bs3-customization#theme-customization
Regards
Gardner.