I'll notify on the following post for any future updates:
https://plus.google.com/u/0/100034615808519099110/posts/Pip36seWtCq

Left-aligned option courtesy of Antons Kijaņica (https://plus.google.com/u/0/114881017441162269480/posts/ZCvBubuQMzh).

Some issues with Chrome. New post form does not align correctly for some reason. In addition, at least some versions of Chrome and Stylish implement the code incorrectly. If the style does not seem to work, open Stylish and choose to edit the style. If the first section lacks the class declarations, replace the part with the first section from the code:
.kDq7F, .RbrTP {
width: 85% !important;
max-width: 1200px !important;
min-width: 520px !important;
margin: 0 auto !important;
}

If you want even wider stream, increase the max-width value for the first two sections, stream width (.kDq7F, .RbrTP) and post input (.EyKftc).

!!
Update 21.5. 10:00 GMT
- Hide "Read more"/"Show less" texts with automatically opened posts (via Shane Corning)
Update 20.5. 16:40 GMT
- Added options
Update 20.5. 6:25 GMT
- Fixed menu from covering content with certain window sizes.

17.5. MIDNIGHT MAJOR UPDATE:
- Icon-only menu: The menu as it was took quite a bit of room, so I decided to make it more like it used to be; good old Google plus.

Let me know if the small menu breaks something with larger resolutions. I only have laptop with low resolution at home so I cannot test it.

17.5. UPDATE:
Evening-update: Fixed the menu from covering the bar that appears on people's profiles

- NOW SCALES WITH THE WINDOW!
- New posts indicator location fixed
- Improved menu location and shadow to fit better with rest of the site
- New post input form box behavior changed. Now it keeps its size and location fixed instead of jumping to side.
- Added instructions on how to disable the always-visible menu
!!

Still needs some little tweaks to make the new post -input box to scale with the rest of the stream. Currently differs slightly. Nothing too major and does not affect the functionality, mainly just a cosmetic thing.
While searching, box alignment is slightly off as well.

Accessibility-option needs to be turned on for What's hot and other pages to function correctly.

Note: Supposedly does not work with Greasemonkey.