If you ask me, the post created by @winteriscoming appears to be starting a bit of a customization revolution.
With Tracking & Analytics (premium sites only) you can customize your forum even more.
Make sure that all custom embeds are set to the below:

To remove the sidebar and widen the post area:
<style>
.qwUf2
{
display: none;
}
.app-desktop .FLyif
{
max-width: 100%;
}
._1wZij
{
max-width: 100%;
}
.app-desktop ._1NkaF
{
max-width: 100%;
}
</style>
To move the like button to the right:
<style>
._3jepg
{
padding-left: 700px;
display: content;
}
.GcHCJ
{
padding-left: 700px;
display: content;
}
</style>
To remove the blue outline when creating/editing a post or comment:
<style>
._1Fqrx :focus
{
outline: 0px !important;
-webkit-appearance: none;
box-shadow: none !important;
}
</style>
To change the selected text colour:
<style>
::selection
{
background: #003cff;
color: #ffffff;
}
</style>
To change your cursor:
<style>
body {
cursor: crosshair;/* or whatever other icon you want. */
}
</style>
To change the scrollbar:
<style>
::-webkit-scrollbar-thumb
{
background: #003cff;
border-radius: 50px;
}
::-webkit-scrollbar
{
background: #003869;
}
::-webkit-scrollbar-track
{
background: #003869;
}
</style>
Feel free to add tweaks below. A few of these that I've added were inspired by @winteriscoming.
@Robin Griggs Wood
Thanks for looking into the idea of creating a zoom feature for the image viewer, @Vytenis Butkevičius. Much appreciated.
@venko gligorov is right, Vytenis, your suggestion for a workaround is totally unprofessional ... 1) for sites where your Wix Forum paid customers are charging their users for content/service 2) for sites where the forum is an added feature to their service, it is not OK to ask your user’s to go through so much complexity to view an image properly.
@Vytenis Butkevičius — just to give you fuller info about: your comment
This is what “full size” looks like in the practical usage of the Image Viewer. It’s not ready for a professional forum at all yet. “Full size” is smaller in the Wix Forum image viewer without a zoom option. This is the make or break point in a photography teaching website.
Without the redundant sidebar the forum dev team added, images were of sufficient size within the forum post itself, without having to click on anything, I might add. With the recently added redundant sidebar, we can no longer give proper feedback to our students. The Wix forum dev team has destroyed my business.
PLEASE, give us the option to turn the forum sidebar off.
Guys, you shouldn't try to change forum styling with custom css. This is not officially supported and our css class names are auto generated and can change at any time. Any custom change can and most likely will result in unexpected results.