With VSD you are supposed to use it throughout. Any tweaks of code outside the programme will be lost when you reopen your project in VSD. You cannot edit the code within VSD. Also, you cannot open a project made in some other application in VSD.
Having said that, you can add
code to VSD using a 'HTML tool', and what you add that way will not get lost when reopening. Some people use it to create an external CSS file and insert a reference to it in VSD. Since I'm a coder, I don't use VSD myself, but I have helped other people with css files for sites created with VSD, an example of that could be this site: http://nqpdesigns.com/
I know of people who actually code most of the page in some other programme and use the HTML tool to insert a more or less ready made site, but for such a purpose I'd recommend a html coding programme, like CoffeeCup's HTML Editor. If someone knows how to code, it's actually not necessary to use VSD.
There are other users in this forum who are real experts on VSD, they will probably chime in here.
Anyway, I'd recommend that you download VSD trial version and play with it to see what you can do with it.
Good luck anyway!