The Everywhere add-on uses this theme file:
/components/com_jreviews/jreviews/views/themes/default/everywhere/reviews.thtml
If you want to customize it, first copy it to your custom theme in jreviews_overrides as shown in JReviews Code Overrides:
/templates/jreviews_overrides/views/themes/{your_theme}/everywhere/reviews.thtml
Individual reviews in Everywhere add-on use the same theme file as reviews of JReviews listings:
/components/com_jreviews/jreviews/views/themes/default/reviews/review_layout.thtml
Copy it to the custom theme before customizing:
/templates/jreviews_overrides/views/themes/{your_theme}/reviews/review_layout.thtml
Remember to clear the File Registry in JReviews administration every time you add a new theme file to your custom theme.