CSS is major factor behind the beautiful website. It is very important to differentiate between the functionality, elements and inherited files of your web-site. There are certain rules and standards when it comes to web development as we go through major project these standards remind us how important and helpful these are.
CSS are of 3 types:
1. Inline CSS.
2. Internal CSS.
3. External CSS.
External CSS will help you add standards to your elements. It should be general so that we don’t need any modifications from end developer to these files. These files should be secure and for a particular layout we should not allow developer to modify these for a page configurations.
Internal CSS helps to define standards for a particular page you should use these to add same parameters to elements occurring together on one page, never use these type of CSS outside a page. It helps to maintain elements standards and look for each page and it helps to identify look of group of elements which are close to each other.
Inline CSS, you can write it along with your HTML code but this type of CSS is highly not recommended as it can create mobile responsiveness problem for your site. You should only use it to align your text or moving a whole block from one space to other. Always care about you empty space when you apply inline CSS.
As far as any external changes I would suggest you to use internal CSS at most when you create something.