Look at that volume icon. This is preschool level, what's going on here?
To me this looks like a race condition between loading certain indicators and applying the theme.
So you end up with a mix of unthemed and themed icons.
This happens to me on 16.04 sometimes with the weather icon.
As soon as the weather changes, that icon goes back to its proper themed look.
I have never seen this happen before on 16.04. Ever.
That said, I went to bed and when I woke up, indeed the icon for the sound applet was once again properly themed. It’s still ridiculously huge though, any idea how to get the old behavior again where the icons are standard sizes and don’t suffer the kinds of race conditions you mentioned?
Try one or more of the following two commands in a terminal after logging in, if this problem arises:
killall caja
and/or
killall mate-panel
If the killall caja command fixes it, then you need to make a script with the following content:
sleep 5
killall caja
Save it as “reset-caja.sh” and then right-click the resultant file and set permissions so that it is executable. Then include it in your start-up applications list
If the killall mate-panel fixes it, do the same thing with it and include the script in the start-up list. The content of this script would be:
sleep 5
killall mate-panel
You might get away with a sleep command shorter than 5 seconds. But, in the first instance, try that and, if it works, go back onto your script and keep reducing it by a second at a time until it stops working. At which point, increase it back up one second.
Unless you’ve manually restarted/replaced the mate-panel, calling killall mate-panel should automatically restart it.
That said, I’ve not experienced the issue that the OP is describing, so I’m not entirely sure 
Ah…i didn’t know that. Thanks for the correction
Yep, just tested it. I will amend my instructions
There is also, I think, the possibility of re-starting the theme via terminal. But, that may be slightly more complicated. So, see if those previous commands work first.
I myself discovered this behavior recently and was quite surprised at it.
Also i wonder (but I have yet to experiment with this), but there is a property that you can set in a program’s .desktop file: X-MATE-Autostart-Delay=5 to delay its start by a few seconds (5 in this example)
Moving on, any way to fix the supersize icons? They're breaking the panel by resizing it and it shows up in inconsistency between icons and in other areas.
(I will remember the other advice when\if the issue shows up again with incorrectly themes icons. Now trying to get properly sized icons.)
I appreciate those who replied earlier but I’m moving back to 16.04.
PlayOnLinux won’t show up in the menu, Gdebi is giving me strange behavior (not bringing up the box to input my admin password to authorize installation) and outside some of what was posted here, I see multiple threads being ignored over the same panel issues. I don’t want to fight on my Laptop, I just want it to work when I grab it. It’s my daily driver.
I wouldn’t have even tried to upgrade to 17.10, but I was hoping this strange graphical glitch which only seems to affect certain applications would be fixed (and it was) but I’ll put up with the glitches over having my environment looking ugly. It’s not just the icons–the panel is all screwed up, panel wallpapers don’t work, there’s a lot going on here that looks ugly and no one seems to be trying to fix them. Instead we have efforts to implement things the community specifically said they didn’t want when asked.
I don’t know what the thinking is here–if we break the traditional desktop people will stop liking it?–but it’s not working. I still like the traditional desktop. It’s the whole reason why I’m here. Remove that and I go too.
I don’t know how long I can hold on with 16.04 LTS, but if these things aren’t fixed in 18.04 LTS I for one will not be upgrading to it.