[Refactor] Improve navbar stylekit

[FlorianJacta]

📌 Description

The original navbar is not very attractive. The goal is to improve it without using CSS and make it a default.

This comes with a plan to improve the overall stylekit.

->

/* Change the color of the tabs */
.header .MuiButtonBase-root.MuiTab-root{
    border: 1px solid var(--color-primary); /* Thin line with 1px width, change color as needed */
    border-radius: 25px; /* Adjust this value for more or less rounding */
    padding: 10px; /* Optional: adds padding inside the rounded border */
    margin: 10px;
    text-transform: none;
    width: 12%;
}

.header .MuiButtonBase-root.MuiTab-root.Mui-selected{
    background-color: var(--color-primary);
    color: white;
}

📜 Code of Conduct

• I have checked the existing issues to avoid duplicates.
• I am willing to work on this issue (optional)

✅ Acceptance Criteria

• The refactored code maintains existing functionality without breaking changes.
• Any new code is covered by unit tests.
• Code coverage remains at least 90%.
• Performance improvements are documented, if applicable.