Thank you for reaching out. In Dreamweaver CC and latest versions, the CSS dialog box has been replaced by the designer. Please check this article: How to create CSS rules in Dreamweaver
Please let me know if you need more assistance on this.
In 2017, Dreamweaver CC was re-built with a new UI and code editor based on Brackets. It's not possible to roll back to the old UI unless you revert to CS6.
Personally, I'm not a huge fan of CSS Designer panels. I think code hints and auto code completion makes working directly with code a lot more efficient than panels.
NEVER edit Bootstrap CSS code. That should be a read only file and off limits to you. If you require custom styles and you don't yet know how to work with SASS/SCSS, then use a separate external stylesheet.