Align "Details" Section Design with Invoice Preview and Add Unified Theme Selector

[RAj5517]

Is your feature request related to a problem?

The "Details" section lacks the same polished design as the invoice preview, making the interface feel inconsistent.

Describe the solution you'd like

Update the "Details" section to match the invoice preview's style (fonts, colors, layout).
Add a theme selector that updates both sections simultaneously (light/dark mode, etc.).
Ensure responsive design for consistency across devices.

[RAj5517]

@ypratham I want to work on this problem, so, if this problem concerns you, please assign me this. I can also help improve the overall issue

[ypratham]

@RAj5517 you can go ahead and work on this, if u are available now?