nextcloud-breeze-dark/core/css/apps/official/_mail.scss

/* Mail --------------------------------------------------------------------- */

.app-mail #app-navigation {
    
    .folders .ui-droppable-active {
        background-color: inherit;
    }

    #app-settings .app-settings-hint {
        color: $foreground-inactive;
    }
}

.app-mail #app-content {

    .setup-content .toggle-manual-mode {
        opacity: 0.7;
    }

    .app-content-list-item.active .app-content-list-item-details {
        opacity: 0.7;
    }

    .app-content-details {

        #mail-message-header {
            background: $background-normal;
        
            & .transparency {
                color: $foreground-normal !important;
        
                a {
                    color: $foreground-link !important;
                }
            }
        }
        
        #mail-content {
            background-color: $background-dark;
            color: $foreground-normal;
            padding: 1em;
            border: 1px solid $decoration-normal;
            border-radius: 3px;
        }

        #mail-content iframe {
            background-color: #EEE;
            padding: 1em;
            border: 1px solid $decoration-normal;
            border-radius: 3px;
        }

        input.to, input.cc, input.bcc, input.subject, textarea.message-body {
            border: none;
            border-top: 1px solid $decoration-normal;
        
            &:hover, &:focus, &:active {
                border-color: $decoration-normal !important;
            }
        }

        textarea.message-body {
            border-bottom: 1px solid $decoration-normal;
        }

        .mail-message-attachments .attachments {

            &> div:hover, &> div span:hover {
                background-color: inherit;
            }
        
            .attachment-download {
                border-color: transparent;
            }
        }

        .transparency {
            opacity: 1;
        }
    }
}