modena.css has the following styles that are intended to set styles on the content node of a TitledPane. But the scene-graph of a TitledPane is not .titled-pane > * > * > .split-pane (for example); rather, it is .titled-pane > * > .split-pane. More specifically, it is .titled-pane > .content > .split-pane.

Note that the .titled-pane > * > * > StackPane style does match on the arrow-button, effectively putting padding around the arrow-button. But that should be done with a more specific style.

.titled-pane > * > * > .split-pane,
.titled-pane > * > * > .text-area,
.titled-pane > * > * > .html-editor,
.titled-pane > * > * > .list-view,
.titled-pane > * > * > .tree-view,
.titled-pane > * > * > .table-view,
.titled-pane > * > * > .tree-table-view,
.titled-pane > * > * > .scroll-pane {
    -fx-background-color: null;
    -fx-background-insets: 0, 0;
    -fx-padding: 0;
}
.titled-pane > * > * > AnchorPane,
.titled-pane > * > * > BorderPane,
.titled-pane > * > * > FlowPane,
.titled-pane > * > * > GridPane,
.titled-pane > * > * > HBox,
.titled-pane > * > * > Pane,
.titled-pane > * > * > StackPane,
.titled-pane > * > * > TilePane,
.titled-pane > * > * > VBox {
    -fx-padding: 0.8em;
}