NewCallBox+CallDisplay: UI facelift

This adds a style.css file to allow the use of custom css

Fixes https://source.puri.sm/Librem5/calls/issues/115:
- Making the phone number larger
- Moving the delete button to the bottom
- Rounding all buttons, and making the grid buttons circular
- Making the call button a bit less wide

src/style.css

@@ -0,0 +1,16 @@
+hdykeypad > button, .dial-button, .delete-button {
+  border-radius: 9999px;
+  -gtk-outline-radius: 9999px;
+}
+
+.dial-button {
+  padding: 19px 42px
+}
+
+.delete-button {
+  padding: 8px 12px;
+}
+
+.phone-number-entry {
+  font-size: 24px;
+}