Allow user to use their own style sheet

What does this MR do and why?

Allow user to use their own style sheet

feat(style.*): Allow user to specify theire custom style sheet with CUSTOM_CSS_SHEET environment variable (#39 (closed))

• The Style sheet should be placed inside the ProdManager/static folder

Closes #39 (closed)

Screenshots or screen recordings

MR acceptance checklist

• My code follows the style guidelines of this project
• I ran pylint and other linters for modified files
• I have performed a self-review of my own code and tested it
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• My code needed automated testing. I have added them (this is optional task)