Specific CSS coding questions can be directed to The Springshare Lounge, a forum for LG authors and Springshare staff. Anyone with an LG account can be a member. Because a number of CSS-related questions have already been discussed there, you might want to search the site before starting a new conversation.
There are many freely available resources on the web to assist you in identifying CSS-amenable elements, properties, and values. Some sites are forums where web designers pose questions or problems and solicit answers and solutions. You can test a solution by copying and pasting code from these sites into a libguide. In most cases nothing will bad will happen as long as the code in enclosed in <style>...</style> tags.
Introductions and Tutorials
A Springshare webinar on using CSS with LG2. You'll need to log in with your LG username and password. A caveat: as the title suggests, it presumes quite a bit of prior CSS experience and may be more confusing than helpful for some authors. The Bootstrap Grid section beginning at 16:00 might be useful if you need to adjust column widths for display on differently sized devices.