How does one generally debug CSS and resolve issues when some elements on the page are not appearing as they should? For now, I have to painfully comment out CSS declarations one by one to understand how the styles are getting displayed.
While you can not "debug" CSS, because it is not a scripting language, you can utilize the Chrome DevTools Elements panel to inspect an element & view the Styles pane on the right.
This will give you insights as to the styles being overridden or ignored (line threw).
View > Developer > Developer Tools
(keyboard shortcut:
CTRL + SHIFT + I)
To find errors & warnings, use CSSLint.