'Image' Filter

Hi there guys!
Is someone able to tell me: with this theme is has a ‘filter’ on the
’image option of the home page that darkens the image.
Is there some way to remove this?
My website is:


There’s an option to change the color if you’d like.
But you can also remove it by adding this in a custom CSS plugin:

.header-image::after {
     background: transparent;

Are you able to tell me what custom CSS plugin I need to install in order to do this?

Any plugin you want. This one for example: https://wordpress.org/plugins/simple-custom-css/