Question: What kind of customer portal? It highly depends on what kind of stuff you want to put there.
As a good foundation, I use HTML5 Boilerplate CSS, as it has a nice normalization, no style and is really light-weight that way. Do not use the templates, though. Try to build without jQuery, but if you have to use complex selections, do not hesitate to use it.
In terms of knowledge, do not miss out on extended BEM / BEMIT. You really should read csswizardry.com and Smashing Mag . As for responsiveness, you might want to check out this device agnostic way .
Do you need a lot of widgets? Then you might want to take a look at Flux and React, maybe combine it with backbone for data locality.
Do you need to reuse your HTML a lot (e.g. lists, blog entries,...), I highly recommend Polymer. You can either use the paper elements in a design-neutral way or create entirely custom elements.
Do you need icons (everyone needs them :D ), go for FontAwesome.
As for the backend, I recommend using whatever you are familiar with (Node.JS, PHP, Python,...), but do_not_use_nosql. NoSQL is nice in terms of BigData and some caching solutions, but for a portal you want to stay with SQL as it enables you to use relational databases.