First you should setup a child theme (https://codex.www.ads-software.com/Child_Themes) because you’ll be needing custom code and CSS.
Inside your Child Theme folder (fluxipress-child) you’ll need to copy and paste the single.php file from the Parent Theme folder (fluxipress).
Inside that file you’ll see this code:
<div class="post-content clear">
<h1><?php the_title(); ?></h1>
<?php the_content(); ?>
You’ll need to insert this code where you want the Date to show up, such as after the title or after the content:
<p id="fluxipress-child-date">Posted: <?php the_time('l, F jS, Y'); ?></p>
I gave the <p> a unique ID so you can style it – that can be whatever you want, as long as it’s not a class or ID that’s being used elsewhere.
ALSO, if you want to format the date differently check out this: https://codex.www.ads-software.com/Formatting_Date_and_Time
Finally, add any styling you want in the new Child Theme’s stylesheet- not the Parent stylesheet.