That is exporting as a PNG, that is the default for a new export.
If you want a PDF, just change the file extention from .png to .pdf Gimp will sort out the correct export dialogue depending on the extension (.jpg / .tif / ....etc)
Edit: changing the Gimp 2.10 theme is in Edit -> Preferences -> Theme and one in there for the icons as well.
If you want a PDF, just change the file extention from .png to .pdf Gimp will sort out the correct export dialogue depending on the extension (.jpg / .tif / ....etc)
Edit: changing the Gimp 2.10 theme is in Edit -> Preferences -> Theme and one in there for the icons as well.