Hello
I assume you are using the Discourse Air Theme.
Something like this, you can achieve it.
Common CSS
body.archetype-regular,
body.archetype-banner,
body.archetype-private_message {
#main-outlet {
margin-top: 30px;
margin-bottom: 0px;
background: transparent !important;
box-shadow: none;
padding: 0em 0em 5%;
width: 100% !important;
#topic-title {
margin-bottom: 0;
background: var(--secondary);
padding: 3em 3em 1em;
border-radius: 1em 1em 0 0;
}
.container.posts {
margin-bottom: 0;
background: var(--secondary);
padding: 0 3em var(--below-topic-margin);
}
.topic-above-footer-buttons-outlet.presence {
margin: 0;
padding: var(--below-topic-margin) 0;
background: var(--secondary);
}
#topic-footer-buttons {
max-width: 100%;
background: var(--secondary);
border-radius: 0 0 1em 1em;
margin: 0;
padding: var(--below-topic-margin) 3em 1em;
}
.more-topics__container {
border-radius: 1em;
background: var(--secondary);
max-width: 100%;
padding: 3em;
}
}
}