How to fix widget style issue

Please make sure you're removing existing widgets from 'Appearance > Widgets' before importing demo data.

Figure -1

If you forgot to delete those widgets from 'Appearance > Widgets' before importing, then it'll be look like below image (Figure -2 image) on your 'Blog' page but you can delete those widgets even after import too from 'Appearance > Widgets' as per 'Figure -1' image.

Figure -2