If you apply custom styles (CSS) to your visuals you always want to be careful as the CSS is shared across the entire HTML page. So CSS applied to one visual could impact other visuals on the page.
To avoid this, you want to do is define a class for the visual, and then use that in your CSS selectors. This way you limit the impact to those classes ((see below for example))
Two other pages that might be helpful:
- Refer to this how-to guide that shows where to define the custom class for the visual: How to modify the font size of the first column of table
- You can also create rich text visuals as well. This sometimes comes in handy to create icons or headers/footer visuals. Adding custom text & footnotes to dashboards