Your site is loading 20 css files including the tabby.css file so it is not easy to troubleshoot what is happening on your site.
Customising the plugin's css without unhooking the original tabby.css file is not supported as stacking new rules on top of the original ones will cause problems when the plugin is updated if the deafult css is changed (this will happen in the next version).
The placement of the first tab can be changed by a value given to
padding-left for the
.responsive-tabs .responsive-tabs__list element.
If there is no padding-left for that element and adding padding-left: 0 has no effect, the default padding value for list items in your theme is overriding the tabby defaults, so you need to add precedence to the rules for the tabs one way to do this for your theme might be by adding .post-body before each rule in a customised version of the rules.