if you are wearing leggings to work the shirt needs to cover your butt and front because camel toe is real haha. also you can wear whatever underwear if your butt is covered but if it's not then you would be safe with a thong. preferable a darker one leggings are generally see through when you bend over
It totally depends on the job and dress code. I wouldn't wear that to my office job but I wear them to my 2nd job at a restaurant. It's best to wear a thong or nothing, anything else risks panty lines.