fixed color of past event heading, faq heading and header-wave for da…

[LalitChauhan27]

Related Issue

• Heading of past events and FAQ section is not clearly visible after enabling dark mode. Heading of past events and FAQ section is not clearly visible after enabling dark mode. #100

Proposed Changes

• Fixed color for past event and faq heading when dark mode is on.
• Fixed header wave color when dark mode is on.

Checklist

• Documentation of the code
• Add your name to the CONTRIBUTORS file
• Testing your code in the local machine

Screenshots

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