How Do I Change The Header Title Size and Colour?

The developers and users of your theme would be the best people to ask. You can reach them here:

https://www.ads-software.com/support/theme/magazine-prime

Hello @kev12345

You can achieve this by applying following CSS code.

.site-branding .site-title {
    display: block;
    font-size: 30px;
    font-weight: 700;
}

.site-branding .site-title a {
    color: #434343
}

.site-branding .site-title a:hover {
    color: #b71b1b;
}

.secondary-bgcolor {
    background: #3636e6;
    color: #fff;
}

Hope this will help.

Thanks.

Great! Thanks everyone!