Category title and description

Hi, I have just started using the Hatch theme (or a child of). I am getting to grips with categories to display images in a grid layout, which is working nicely. However I want to add the Category title and the description to the page.

I have figured out that it’s the archive.php file that this page uses, and have added the following code:

<?php single_cat_title( ”, true ); ?>
<?php echo category_description( $category_id ); ?>

Bingo, the title and the descriptive text are now showing! If this code is incorrect – please advise!
The title however is not showing in H3 text that is light grey – as per all the other titles on my other pages. I have tried adding an <H3> tag and it then appears as the correct size, but not in the grey.

Do I need to include some css somewhere, or include an include file?

I have a fairly rusty understanding of coding, but usually can muddle through, but I’m lost with this one – please help!!

My site is currently running locally on my Mac, so I guess I can’t share the link. I want the title to look like it does here:

https://hatchdemo.wordpress.com/full-width/

Any help would be greatly appreciated.