Hi Kai,
I have checked again your site and the tab headings are showing in bold on the frontend of the site just fine.
This is what loads by default:

And this is when I disable the bold style through the code inspector:

I have asked two colleagues in different locations to check this and it loads as bold on their end as well so I am not really sure what is causing it not to display correctly on your end. I have tested this on Chrome and Firefox and they display with the same font weight.
You might want to try to load this in the functions.php file of the child theme to make sure that the bold font weight is loaded:
/**
* Enqueue scripts and styles.
*/
add_action( 'wp_enqueue_scripts', 'add_scripts' );
function add_scripts() {
wp_enqueue_style( 'custom-google-font', 'https://fonts.googleapis.com/css?family=Open+Sans:400,600,700,800' );
}
Then try setting the font weight to 700.
This makes sense. I have already added this in our request list for our developers to consider.
With style guide do you mean a list of selectors that you can use in the custom CSS? If so, we do not have this as this is something too broad to collate. What I can suggest is learning how to write CSS selectors so that you can target specific elements and style them through CSS accordingly. If you are using v2 elements, there is a less need to write custom CSS as v2 elements contains a lot of option that you can set and will generate the relevant CSS to the element automatically.
Hope this helps.