(core) Fixes date filter calendar view for dark mode

Summary:
The calendar picker does not support dark theme just yet. So the diff
makes sure the that the background remains white.

Test Plan: Should not break anything.

Reviewers: cyprien

Subscribers: anaisconce

Differential Revision: https://phab.getgrist.com/D3744

app/client/ui/ColumnFilterCalendarView.ts

@@ -161,4 +161,5 @@ const cssLinkRow = styled('div', `
 
 const cssDatepickerContainer = styled('div', `
   padding-top: 16px;
+  background-color: white;
 `);