Disable header image highlight

Is there in your style.css file of your template a CSS code lookslike header-image hover? Or something like that. If so, you have to try to delete that one and have a look if it has been changed.

The theme is using this CSS to set the hover opacity:

.header-image a:hover img {
    opacity: 0.85;
}

You can use this custom CSS to override it:

.header-image a:hover img {
    opacity: 1;
}

To add CSS: If you are using WordPress 4.7, use the “Additional CSS” option in the Customizer. If your theme has a custom CSS option, use that to add the CSS. If not, install a plugin like https://www.ads-software.com/plugins/simple-css/ or https://www.ads-software.com/plugins/custom-css-js/. Or, if you have Jetpack installed, enable its Custom CSS module.

Thanks so much guys. Used the simple CSS plugin. Worked like a charm. Is it possible to do the same with a post title, for example, Hello World on my site gilesjmblackley.com ?
It highlights when you hover the mouse over the text.

Thanks again,

Giles

/* override article title hover color */
.entry-title a:hover {
    color: #000;
}

If you’re wanting to make additional changes to your site you should become familiar with utilities like Firefox or Chrome Developer Tools. They let you see the site structure and applied CSS, and make test modifications in “real time” to see the effects.
https://developer.mozilla.org/en-US/docs/Tools
https://developer.chrome.com/devtools

Here’s a reference on CSS:
https://premium.wpmudev.org/blog/css-mega-guide/

I’m not suggesting that you can’t, or shouldn’t, ask for help here if you need it; that’s what the forums are for. The volunteers here will appreciate it if you have at least a basic understanding of what you’re working with before you post.