I could isolate the arrow issue down to the svg-files. For some reason RT wants to read the svg-arrow-files, which Windows can’t read. Removing all svg-images solves it. But there is still a different. RT is loading now the 24 x 24 arrow.
To solve the whole issue, delete every icon size and scalable folder except the 16 x 16 folder.
@TooWaBoo,
What is going to happen to the TooWaBlue Themes? Are you going to change to Gtk 3.18 so they will appear in the dropdown? Just asking, not a priority.
There’s a naming issue in the current download of RT5 Gtk3.18.
To fix it by yourself, rename the file TooWaBlue.css to TooWaBlue-GTK3-_19.css.
…or download the RT5 Gtk3.22 version from here:
https://drive.google.com/drive/folders/0B2q9OrgyDEfPS2FpdDAtMVI1RG8
…but there is bug in Gtk3.22. Some curves will not fully expand and you have to hover any button after expanding to fully expand them. Thats why it’s not the recommended version, but I like the Gtk3.22 version more (Looks nicer 
)
No one had raised the problem before that builds using GTK+ <=3.18 don’t show the TWB theme (by design, but it’s a new design) so unfortunately they’re not visible when RawTherapee is built using <=3.18. This also means that nobody tested those themes in <=3.18, only in >=3.20. @TooWaBoo before we add that to the git repository, have you verified that it works as expected in <=3.18?
The theme is compatible with Gtk3.18 and was used by many users since a Gtk3.18 version was available. e.g. @heckflosse
The name convention was established a couple of weeks ago, so in the past there was no need to use a special name. Before Gtk.3.20 I was using it with Gtk3.18 the whole time.
@TooWaBoo theme handling code was committed two months ago in commit 55759c04. As I haven’t seen anyone mention renaming themes in this forum, nor has anyone complained of missing themes since then, therefore, unless users figured out that the theme had to be renamed on their own (which is unlikely), I conclude that the theme has not been tested by users on <=3.18 since then, and much changed in the last two months.
It must be compatible with GTK+ 3.20-3.22, otherwise the file name is wrong. I assume this is a language barrier misunderstanding.
Before I commit, please confirm that TooWaBlue version 2.40 was tested using GTK+ 3.18. Additional points if it was tested using GTK+ 3.16.
TooWaBlue v2.40 is compatible with Gtk3.20-3.22
TooWaBlue v1.94 (That’s what we’re talking about right now) is compatible with Gtk3.18
But this version has only the name TooWaBlue.css and that’s why it is not shown in the preferences in the current Gtk3.18 installation.
Great, where do I get 1.94 from?
When I install the current RT5 from the download page, it’s in there (with the wrong name).
TooWaBlue.txt (21,0 KB)
@TooWaBoo both TWB and TWB-D in 5.0-gtk3 are version 2.40.
If you see anything different, it’s a packaging issue - ping @sguyader.
Regardless, we must still commit 1.94. I’ll do that later, busy with the versionfix branch.
Hi @Partha,
You may want to include librsvg-2-2.dll in the next portable build for those who don’t have it.
Version 1.94 was in the Gtk3 branch until January and then suddenly was gone.
Installation screenshot of RT5 Gtk3.18
Don’t forget the image folder. It is missing too.
Thank you very much for all the great work that you folks have done so far! 
This could have been made clearer:
Update TooWaBlue Gtk+3.22 Version 2.31 · Issue #3604 · Beep6581/RawTherapee · GitHub
Is 2.40 compatible with GTK+ 3.18?
No, it’s not.
No, it’s impossible. Hombre had the same problem with the RT theme and did something hardcoded.
I mean remove the CSS code which sets custom images for standard widgets, so we don’t re-add and ship stuff which is only for a legacy GTK+ version.
If I do this, i’ll get the original checkbox of Adwaita with the green background. This was the reason, why I needed the external images.
Will be fixed in 5.0-r1.