modal dialog classes(CSS) location

[null null]

Could you please let me know where are the classes for the Show modal dialog stored as  css.

I want to change the css for the following classes

.ms-dlgTitleBtns, .ms-dlgTitleBtns

thanks

[#2]

These will be on corev15.css (/_layouts/15/1033/styles/Themable/corev15.css) ... My recommendation wouldn't be to modify the file but to overwrite them using your own css. 

[#3]

I am working on SharePoint online. How do i edit the file

and whats the best practise as it appears to me as if the css is wrong and i dont know how it got changed.

thanks

[#4]

You should overwrite this CSS node with your own css file placed on a CDN or the assets folder. With a custom action deployed by PS you are able place your linked css file on every page.

[#5]

You can use @Torsten Schuster's answer, or you can add your own CSS file reference to the layout page (if you are using one throughout the site collection(s)) or even on the master page. If you need to modify the modal on just a couple of pages, then add it using a Script Editor webpart.