Fix color palette

Currently the color palette between the login screen, navbar and home page is all the same (or similar).

Not super high priority for now, but this involves fixing the css properties and bootstrapping correctly.

Look at the CSS files for the login screen, navbar, and home page components. Identify the color variables or classes being used. The goal is to define a consistent color palette and apply it correctly across these components, ensuring Bootstrap classes are used appropriately if the project uses Bootstrap.

索引モデルが issue の本文から書いたものです。