By "light grey colored bar behind", I believe you're wanting a text background like the screenshot below?
If so, and because you're not using the Cornerstone builder, you need to use the Headline Shortcode. For more details about the Headline Shortcode, please see https://theme.co/apex/forum/t/shortcodes-custom-headline/158
Your shortcode setup should have inline styles like the following:
[custom_headline level="h2" style="text-align:center;background:lightgray"]Custom Headline[/custom_headline]
Take look at the
style attribute values in the sample shortcode. That is called inline CSS. You can add more CSS in there but please just note that we expect that you know CSS. If you don't know how to use CSS yet, please learn from sites like https://www.w3schools.com/css/default.asp
In case you don't want to learn CSS, I'd recommend you rebuild the page using Cornerstone and use the design options of the builder.
Hope that helps.