added the brand style colors

[jeromehardaway]

This pull request introduces a new theme and custom styling for the Streamlit application. The changes include setting up brand colors, configuring page settings, and defining custom CSS for various elements to align with the Vets Who Code branding.

Theme Configuration:

• Added a new theme section in .streamlit/config.toml to define primary, background, secondary background, and text colors, along with the font. (.streamlit/config.toml)

Page and Styling Configuration:

• Configured page settings such as title, icon, and layout in app.py. (app.py)
• Added custom CSS in app.py to style various elements including inline code, headers, sidebar, buttons, chat messages, sliders, expanders, chat input, success and error messages, and download buttons to match the Vets Who Code brand colors. (app.py)