@-moz-document url-prefix(about:blank), url-prefix(about:newtab) {
    html:not(#ublock0-epicker), html:not(#ublock0-epicker) body, #newtab-customize-overlay {
        background: #FDF6E3 !important;
    }
}

@-moz-document url-prefix(https://app.slack.com) {
    body, .p-client, .p-message_pane, .c-scrollbar__hider, .c-message--hover, .c-message, .c-message--light, .p-workspace__input, .p-workspace_input {
        background-color: #FDF6E3 !important;
        color: #556b72 !important;
    }

    .c-scrollbar__hider::before, .c-message_list__day_divider__label__pill, .p-message_input_field, div[role="presentation"], .c-search__input_box, footer {
        background-color: #FDF6E3 !important;
        color: #556b72 !important;
    }

    * {
        color: #556b72 !important;
    }

}

@-moz-document url-prefix(about:blank), url-prefix(about:newtab) {
    html:not(#ublock0-epicker), html:not(#ublock0-epicker) body, #newtab-customize-overlay {
        background: #FDF6E3 !important;
    }
}

Remove slack specific css styles
Remove slack specific css styles
