Changes for slider colors.

JMetroBase.css

/*******************************************************************************
 *                                                                             *
 * Slider                                                                      *
 *                                                                             *
 ******************************************************************************/

.slider {
    -fx-skin: "impl.jfxtras.styles.jmetro8.SliderSkin";
}

.slider {
    -show-value-on-interaction: true;
}

.slider .thumb {
    -fx-background-color: accent_color;
    -fx-background-insets: 0;
    -fx-background-radius: 6px;

    -fx-padding: 12px 4px 12px 4px;

    -fx-effect: null;
}

.slider:vertical .thumb {
    -fx-padding: 4px 12px  4px 12px;
}

.slider .track {
    -fx-background-insets: 0;
    -fx-background-radius: 0;
    -fx-padding: 0.083333em; /* 1 */
}

.slider .fill {
    -fx-background-color: accent_color;
    -fx-padding: 0.083333em; /* 1 */
}

.slider:hover .thumb{
    -fx-background-color: thumb_hover_color;
}

/* pressed */
.slider .thumb:pressed {
    -fx-background-color: thumb_pressed_color;
}

/* slider disabled */
.slider:disabled {
    -fx-opacity: 0.35;
}

/* Slider popup showing its value */
.slider-popup .label {
    -fx-background-color: #f2f2f2;
    -fx-border-color: #cccccc;

    -fx-text-fill: #111;

    -fx-padding: 0.5em 0.75em 0.5em 0.75em;
}