This is a traditional Japanese hand technique called Sashiko uses a simple running stitch to create beautiful patterns by hand!
WonderFil Specialty Threads - Sashiko Embroidery by Hand Using
Sashiko Embroidery Technique Tutorial – the thread
Essential tools for sashiko stitching - Stitched Modern
WonderFil Perle Cotton Thread Box (Stars)
Sashiko Embroidery Technique Tutorial The Thread
Sashiko Embroidery Technique Tutorial The Thread
WonderFil Specialty Threads - Sashiko Embroidery by Hand Using
sashiko — Blog - WonderFil
Beginner's Guide to Sashiko Embroidery – Cloud Craft
Short Pitch Variegated Sashiko Thread - Blues (#98)