I am so glad to see sewing theme parties on the rise! Here is older, but still fresh!, Frosting for sewing themes here.
...here are a few more!
Use embroidery hoops with fabric make a great backdrop for your party! Source
...you could use the hoops with chalkboard paint as well! Source
Use simple craft letters and wrap in yarn! Spell out initials, name or something fun like "sew" or "craft". Source and tutorial
Add some drizzles of chocolate to your cake balls... not only will they look like yarn balls - but it will cover up any imperfections ;o) Source
...another pop idea... these spools of thread! Source and tutorial found via maddycakemuse