I added some underline styles to the texts with links but the read more button has the colour as well. How could I disable the colour underneath the read more button?
use this CSS instead, it excludes the button class from your styles:
.inside-article p a:not(.button):link, .inside-article p a:not(.button):hover, .inside-article p a:not(.button):active, .inside-article p a:not(.button):visited {
border-bottom: 4px solid #FFFFCC;
}