styles.css

/* Prevent the menu from protruding from the editor if really many styles are added to the menu. */
.editor_atto_menu .atto_styles_button .dropdown-menu {
    max-height: 200px;
    overflow-y: scroll;
}

/* Prevent the menu from wrapping the menu items */
.editor_atto_menu .atto_styles_button .dropdown-menu > li {
    white-space: nowrap;
}

/* Separate the style list menu items more clearly. */
.editor_atto_menu .atto_styles_button .dropdown-menu > li {
    border-bottom: 1px solid #eee;
    padding-bottom: 10px;
    padding-top: 10px;
}

/* Do not add a bottom border to the last list element. */
.editor_atto_menu .atto_styles_button .dropdown-menu > li:last-child {
    border-bottom: none;
}

/* Position menu item icons */
.editor_atto_menu .atto_styles_button .dropdown-menu > li > a i {
    display: inline-block;
    height: 15px;
    margin-right: 10px;
    width: 15px;
}

/* Override Atto core paddings for menu items to prevent the disabled inline menu items from showing a hand cursor */
/* Additionally remove text underline from menu items in this widget because Boost adds underlines to menu items by default
   but for disabled menu items this is counterproductive */
.editor_atto_menu .atto_styles_button .dropdown-menu > li > a {
    padding: 0;
    text-decoration: none;
}
.editor_atto_menu .atto_styles_button .dropdown-menu > li > a span.preview,
.editor_atto_menu .atto_styles_button .dropdown-menu > li > a span.title {
    display: inline-block;
    /* stylelint-disable-next-line declaration-no-important */
    max-width: calc(100% - 55px) !important;
    /* stylelint-disable-next-line declaration-no-important */
    margin: 0 !important;
}

/* Override Moodle core's styling, because the underline should not appear within the dropdown list. */
.editor_atto_menu .atto_styles_button .dropdown-menu > li:focus,
.editor_atto_menu .atto_styles_button .dropdown-menu > li:hover {
    background-color: #f8f9fa;
}
.editor_atto_menu .atto_styles_button .dropdown-menu > li > a:focus {
    box-shadow: none;
}

/* Disabled inline style menu item visually */
.editor_atto_menu .disableinline .atto_styles_button .inlinestyle {
    cursor: default;
}
.editor_atto_menu .atto_styles_button .inlinehint {
    display: none;
}
.editor_atto_menu .disableinline .atto_styles_button .inlinehint {
    display: inline-block;
    margin-left: 39px;
}