Implement Dark/Light Theme Toggle

[OPSergio]

Description

Currently, the application only supports one theme (light or dark). We want to give users the option to switch between a light theme and a dark theme without maintaining two separate versions of the app.

Note:
This should be considered an enhancement rather than a necessity. It improves user experience but is not a critical or high-priority feature at this stage.

Goal

• Add a toggle (switch) to change the theme to dark or light.
• Ideally, use a solution that requires minimal extra maintenance (e.g., CSS variables, Darkmode.js, etc.).
• Ensure that Plotly/Dash charts also switch to a corresponding dark theme (for example, using template='plotly_dark' when dark mode is active).