Twenty Seventeen Theme Adding a Box/banner around/behind site title

body:not(.title-tagline-hidden) .site-branding-text {
border: 1px solid red;
border-radius: 10px;
padding: 5px;
background-color: rgba(201, 201, 201, 0.3);
}

this will creates a border around the title and tagline.

If you want just around the title, and to make font size larger:

h1.site-title{
border: 1px solid red;
border-radius: 10px;
padding: 5px;
background-color: rgba(201, 201, 201, 0.3);
font-size: 3em;
}

play around with the background color and transparency

How would I make just the tagline font size larger or make them both larger but different sizes?

tagline:

p.site-description {
font-size: 3em;
}

Also, how would I make the tagline bold like with my homepage intro where it says Welcome to etc.?

add

font-weight: bold

check out:

https://www.w3schools.com/css/css_font.asp

You are an angel, mate! Thank you!

Looks good

use plugin Contact Form 7 for your contacts page

What’s that, where do I find it? And why?

Use it on your contacts page, create a form that people can use to contact you on

https://www.ads-software.com/plugins/contact-form-7/