Here’s an easy way to show that hidden folder:
1 - In Obsidian… go to Preferences > Appearance > CSS-Snippets (at the bottom).
- Turn the snippets switch to “On”.
- Click the little folder icon
above the switch. A popup will appear (that popup is the folder /vault/.obsidian/snippets/).
2 - Drag your snippets.css file into that popup window.
DONE! 
Your CSS file is now located at /vault/.obsidian/snippets/snippets.css.
- You can open your CSS file in any text editor [like Notepad (Windows) or TextEdit (Mac)] - (a CSS file is just a text file).
- You can add/change styles in that CSS file.
- New changes are automatically pulled in by Obsidian, any time you save the snippets.css file.