JD suggested the solution to this issue. I am documenting it here for others who might face the same issue.
Color palette setting may make the text in dropdown list (<select>) invisible

To reproduce the issue, try Neonize or C8 Dark palette from here.
That’s because the select HTML tag’s background is not defined in the core themes. We need to style it separately, by adding this CSS to any tiddler tagged $:/tags/Stylesheet
select { background: <<colour page-background>>; }
Or if you want the tiddler background color:
select { background: <<colour background>>; }
Or if you want it to be transparent:
select { background: transparent; }
Thanks JD
I have fixed the issue to the best of my knowledge and opened up a pull request at Github