web/web_widget_many2one_simple/static/src/css/many2one_simple.scss

.o_field_widget {
    // Flex fields (inline)
    &.o_field_many2one_simple {
        display: inline-flex;
        > span,
        > button {
            flex: 0 0 auto;
        }

        .o_external_button {
            padding: 0;
            margin-left: 2px;
            font-size: 19px;
            color: #7c7bad;
            border: none;
            &:hover {
                background-color: transparent;
            }
        }
    }
}