When you want to use styles in your content, the style sheet needs to be made available for the content in question. In Flare, you can associate style sheets with a single topic
Applying style sheets at the topic level is attractive because you can associate different style sheets with different topics, even associating multiple style sheets with a topic. However, in most cases you will use a single style sheet for an entire project or target. Therefore, it is usually better to use a master style sheet at one of those levels. That way, you do not need to manually associate the style sheet with a topic before using the styles in it.
See Applying Master Style Sheets.
Note: If you apply a master style sheet at either the target or project level, you will no longer be able to associate style sheets at the topic level (unless you remove the master style sheet associations at the project and/or target level). If you have previously applied multiple style sheets to a single topic and then switch to using a master style sheet, you should make sure that all of the styles you need from the various style sheets are all included in your master style sheet.