Hey Henry,
There are a couple of ways to change the styling on this text but good practice is to add a CSS Class with the properties you want and then assign that class to the element.
Here as an example but you would need to add and edit to the style you want:
I added the following to my style.css page which creates a class we can use on our HTML:
.logotitle {
color: #00FF00;
font-family:arial;
font-size: 18pt;
}
Note: You could name this class anything you wanted but make sure its starts with a . and make sure it doesn't match another class or it will conflict.
Next, I need to assign this class to the element we want to impact. In this case, I want the text from the text we added before to be impacted. I went to the header.hbs and added the following in bold:
<div class="logo">
{{#link 'help_center'}}
<img src="{{settings.logo}}" alt="{{t 'home_page' name=help_center.name}}">
{{/link}}
<span class="logotitle">Your Text Here</span>
</div>
Publishing this would now have my text look like the following"

You can now add/edit the styling properties on the CSS class you created. w3schools CSS section has more ideas on styling properties if you need a reference.
Cheers!