DatePicker-Overlay button (today, cancel) should have assigned theme tertiary

[eforest]

Description

DatePicker-Overlay button (today, cancel) should have assigned theme tertiary.
Instead there's a workaround in vaadin-date-picker-overlay-content-styles.js that tries to mimick that theme.
Unfortunately that (can) cause invisible buttons in case "background-color: transparent" is not "compatible" with default button theme.
(look for comment: /* TODO: Would be great if I could apply the "tertiary" theme from here instead of copying those styles */)

Expected outcome

today-button / cancel-button should have assigned theme tertiary and therefore get styled accordingly

Actual outcome

In my case today-button / cancel-button are completely transparent.

Browsers Affected

• Chrome
• Firefox
• [?] Safari
• [?] Edge
• [?] IE 11
• [?] iOS Safari
• [?] Android Chrome