Contradiction between Milligram and internal stylesheet

I have use milligram as CSS and also a stylesheet. Now webpage is being some kind of mismatch. Some of element use milligram style and some the stylesheet linked.

All I want is to use the Milligram for default and my stylesheet for the element whose classes I have defined explicitly.

That’s not how CSS works. If a a rule matches it will be applied unless overridden by a later or more specific rule.