So I think there is a Windows file that is change when changing the CSS file that prevents opening the gallery.
Modifying the gallery's 'theme.css' file (or updating Windows) should not prevent JuiceboxBuilder-Pro from opening the gallery.
In fact, I have modified your gallery (the one I downloaded earlier) and can open the modified gallery without any problem in JuiceboxBuilder-Pro. (I run the latest release version of Windows 10 Pro 1903 Build 18362.418.)
... everything works fine I can open all my Gallery Hi !!!!
I really do not know what might have caused your problem .
If you had a gallery file open in another program (for example, if you had the 'config.xml' file open in a text editor) whilst the gallery was still open in JuiceboxBuilder-Pro, then there is a chance that JuiceboxBuilder-Pro would not be able to empty its temporary working folder when closing the application but this usually results in the 'Open Gallery...' button being inoperable (rather than triggering an 'Invalid Gallery Folder' error message).
In any case, I am glad to hear that you have been able to resolve your problem. Thank you for letting me know.
For the modification of Button Url I dropped or you have my gallery you can change it and send it to me If possible if no it does not matter.
I have modified your gallery and have emailed you a link to the zip file.
The modified gallery is inside a folder named 'Annee-modele_MODIFIED'.
I have modified three files:
(1) index.html - I added the necessary code to override the Download Button's functionality.
(2) config.xml - I added some configuration options (and changed some others).
(3) theme.css - I modified line 825 to change the icon for the Download Button.
Open the 'index.html' file in a browser and you'll see the result.
Please note that although the gallery will open in JuiceboxBuilder-Pro, the Download Button will still have the regular Download Button icon when viewed in JuiceboxBuilder-Pro's live preview window. (JuiceboxBuilder-Pro reads only the gallery's 'config.xml' file when opening a gallery and knows nothing about the modifications made to the 'index.html' and 'theme.css' files.)
Also, if you do open the gallery in JuiceboxBuilder-Pro and re-save the gallery on the 'Publish' tab, then please note that the modifications will be lost (as JuiceboxBuilder-Pro will overwrite the modified 'index.html' and 'theme.css' files).
I have included the original gallery in my zip file (in a folder named 'Annee-modele_ORIGINAL') so that you can compare the 'index.html', 'config.xml' and 'theme.css' files to see the changes that I have made.
I hope this helps.