(core) Add dark theme to date picker

Summary:
Updates the date picker to use a dark theme when dark mode
is enabled.

Test Plan: Manual.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3780

app/client/ui/ColumnFilterCalendarView.ts

@@ -160,5 +160,4 @@ const cssLinkRow = styled('div', `
 
 const cssDatepickerContainer = styled('div', `
   padding-top: 16px;
-  background-color: white;
 `);