Improve contrast of new theme

[tabesin]

I'm submitting a ... (check one with "x")

• [x] bug report
• [ ] feature request
• [ ] support request => Please do not submit support requests here, instead ask your query in out Gitter channel at https://gitter.im/fossasia/meilix

Actual Behaviour

The new theme lacks contrasts, which makes it difficult to follow on some devices.

Expected Behaviour

Steps to reproduce it

Related code:

insert any relevant code here else remove this section

Other information:

System information:

Your operating system

Would you like to work on the issue?

[rajvaibhavdubey]

I would like to fix this.