[style]: Improve text and button visibility in dark mode in index.html testimonial section

[akshay0611]

What's the style idea?

We need to improve the visibility of the text and buttons in dark mode in the testimonial section of the index.html page. Currently, elements like the "Users sharing their experience" text and navigation buttons are not visible in dark mode due to insufficient contrast between the text color and the dark background.

Add screenshots

Dark Mode Screenshot (before fix):

Dark Mode Screenshot (after fix):

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