Pages like profile, My stuff and discuss still has 2.0 like pages.
are you ever going to change them ST?
If they had time to change some pages, and the whole accent, why did they leave some?

when green flag clicked
wait (plz fix) secs
If you see this comment, and might know, plz reply 

Updating outdated pages to 3.0 is already on the Scratch Team's radar– pages like the front page and project pages were prioritized because they're the most common pages people will see. However, all pages will be updated in due time, it's just that more niche pages such as the profile page and forums will take a little longer.

As someone who's tackled it before, graphic design takes time, and especially for the Scratch Team since they do research and studies to provide the best possible end result.
Think about when they turned the whole site purple; sure, under the hood it's just a few changed hex values, however they did a lot of research to decide on the color purple, and even now they're taking suggestions and feedback regarding the change.

They're working on it.

While it would be relatively easy to make the page look like something else (with the difficulties mentioned in @NamelessCat's post), the ST is also making backend changes.

The Scratch website isn't really one website, with two different looks. It's two- scratchr2 (the 2.0 website), and scratch-www (commonly referred to as the 3.0 website, although it was actually released in 2017, two years before 3.0), and they're kind of mashed together. Making scratchr2 look like scratch-www would only require changing a few CSS files (CSS is the language used in styling webpages), but the backends would be the same as before- and that's not necessarily a good thing, because while both would look like scratch-www, behind the scenes they're still two separate things. The ST wants to update the backend before they update the frontend.

TL;DR:
They're working on it. Be patient.

BigNate469 wrote:

They're working on it. Be patient.

Adding on to that, they just changed the scratchblocks to 3.0, which could mean this will be the next update we see (2.0 -> 3.0)

Please be mindful that updates to the website are a work in progress - there's no need for a suggestion for work that is currently known and ongoing.