I spend a few hours trying to understand this issue. I cannot import a custom theme in a project using bootstrap 4.1.x. Even bootswatch themes do not work. For example, I save this css:
which is the official cyborg bootswatch theme. I import it in Settings>Themes. I then have two possible behavior:
I have an Invalid CSS: This file contains invalid CSS and can't be imported error, or
I can import the theme, but the software does not switch to the new theme or allow me to save the Setting window.
I am using BSS 4.3.4. I tried reinstalling the software and deleting the local data in AppData to no avail. I tried other themes (non bootswatch) with the same results. I made sure to use bootstrap 4.1 themes.
I would like to add that I was able to import my custom theme a few days ago. I just realized that BSS has automatic update. Is it possible that something broke?
More info: It does not work even if I import the bootstrap.min.css exported by the BSS when I export the website. Is there any way to downgrade the software?
I'm having the same problem. I did some themes, but after the 4.3.4 update, the Bootstrap Studio no longer allows saving the theme. It simply ignores and does not save the chosen theme. I can import the theme but do not apply. There is some way to go back to the previous version.