Tomaz implemented a system setting to enable/disable style previews in the Styles and Formatting tab (bug 91495), but this is only accessible through the expert configuration setting '/org.openoffice.Office.Common/StylesAndFormatting Preview' and should be made easily available within the sidebar as a checkbox, similar to this mockup (attachment 115020 [details]).
Susobhan Ghosh committed a patch related to this issue.
It has been pushed to "master":
tdf#93845: Style Preview Checkbox for Styles & Formatting sidebar
It will be available in 5.3.0.
The patch should be included in the daily builds available at
http://dev-builds.libreoffice.org/daily/ in the next 24-48 hours. More
information about daily builds can be found at:
Affected users are encouraged to test the fix and report feedback.