Define yearly-overlay part for styling access

tp-date-picker.js

@@ -164,7 +164,7 @@ class TpDatePicker extends FormElement(LitElement) {
           `
         })}
       </div>
-      <div class="year-overlay" ?visible=${this.showYearSelector} @click=${this.selectYear}>
+      <div class="year-overlay" part="year-overlay" ?visible=${this.showYearSelector} @click=${this.selectYear}>
         ${years.map(y => html`
           <div .value=${y}>${y}</div>
         `)}