How to style menu button and menu items

MenuButton uses Menu internally and has a similar API. In such way that MenuButton contains MenuItems list just like Menu. So I think you need to try to play with .menu, .menu-button and .menu-item CSS selectors in caspian.css. More specifically with .menu-item.

EDIT: It seems you need to change the .context-menu too, because the popped up menu of the menuButton is ContextMenu.

.menu-item .label {
    -fx-text-fill: white;
}

.menu-item:focused {
     -fx-background-color: darkgray;
}

.menu-item:focused .label {
    -fx-text-fill: blue;
}

.context-menu {
    -fx-skin: "com.sun.javafx.scene.control.skin.ContextMenuSkin";
    -fx-background-color: black;
    -fx-background-insets: 0, 1, 2;
    -fx-background-radius: 0 6 6 6, 0 5 5 5, 0 4 4 4;
/*    -fx-padding: 0.666667em 0.083333em 0.666667em 0.083333em;  8 1 8 1 */
    -fx-padding: 0.333333em 0.083333em 0.666667em 0.083333em; /* 4 1 8 1 */
}

This has also been asked here and here, so I decided to write a CSS template for styling menu bars.

Using this CSS template is a very easy way to style a MenuBar, its top-level MenuButton entries, and each MenuButton's MenuItem children, i.e, "the whole menu bar".

The only thing that has to be done is to adjust four variables according to one's needs:

• -fx-my-menu-color: The menu bar's default background color (i.e., when item is not hovered / selected)
• -fx-my-menu-color-highlighted: An item's background color if it is hovered / selected.
• -fx-my-menu-font-color: The menu bar's default font color (i.e., when item is not hovered / selected)
• -fx-my-menu-font-color-highlighted: An item's font color if it is hovered / selected.

The complete CSS file is commented to explain each defined rule:

/* VARIABLE DEFINITIONS: Only these 4 variables have to be adjusted, the rest is copy-paste */
* {
    -fx-my-menu-color: #263238;                  /* Change according to your needs */
    -fx-my-menu-color-highlighted: #455a64;      /* Change according to your needs */
    -fx-my-menu-font-color: #FFFFFF;             /* Change according to your needs */
    -fx-my-menu-font-color-highlighted: #FFFFFF; /* Change according to your needs */
}

/* MENU BAR + Top-level MENU BUTTONS */
/*** The menu bar itself ***/    
.menu-bar {
    -fx-background-color: -fx-my-menu-color;
}

/*** Top-level menu itself (not selected / hovered) ***/
.menu-bar > .container > .menu-button {
    -fx-background-color: -fx-my-menu-color;
}

/*** Top-level menu's label (not selected / hovered) ***/
.menu-bar > .container > .menu-button > .label {
    -fx-text-fill: -fx-my-menu-font-color;
}

/*** Top-level menu's label (disabled) ***/
.menu-bar > .container > .menu-button > .label:disabled {
    -fx-opacity: 1.0;
}

/*** Top-level menu itself (selected / hovered) ***/
.menu-bar > .container > .menu-button:hover,
.menu-bar > .container > .menu-button:focused,
.menu-bar > .container > .menu-button:showing {
    -fx-background-color: -fx-my-menu-color-highlighted;
}

/*** Top-level menu's label (selected / hovered) ***/
.menu-bar > .container > .menu-button:hover > .label,
.menu-bar > .container > .menu-button:focused > .label,
.menu-bar > .container > .menu-button:showing > .label {
    -fx-text-fill: -fx-my-menu-font-color-highlighted;
}

/* MENU ITEM (children of a MENU BUTTON) */
/*** The item itself (not hovered / focused) ***/    
.menu-item {
    -fx-background-color: -fx-my-menu-color; 
}

/*** The item's label (not hovered / focused) ***/   
.menu-item .label {
    -fx-text-fill: -fx-my-menu-font-color;
}

/*** The item's label (disabled) ***/   
.menu-item .label:disabled {
    -fx-opacity: 1.0;
}    

/*** The item itself (hovered / focused) ***/    
.menu-item:focused, .menu-item:hovered {
    -fx-background-color: -fx-my-menu-color-highlighted; 
}

/*** The item's label (hovered / focused) ***/  
.menu-item:focused .label, .menu-item:hovered .label {
    -fx-text-fill: -fx-my-menu-font-color-highlighted;
}

/* CONTEXT MENU */
/*** The context menu that contains a menu's menu items ***/  
.context-menu {
    -fx-background-color: -fx-my-menu-color; 
}