did he propose theories on new elements?
Yes. Theory of Phlogiston in 1777
Carl Wilhelm Scheele
Scheele achieved astonishingly prolific and important results without the expensive laboratory equipment to which his Parisian contemporary Antoine-Laurent Lavoisier was accustomed. Through the studies of Lavoisier, Joseph Priestley, Scheele, and others, chemistry was made a standardized field with consistent procedures. Although Scheele was unable to grasp the significance of his discovery of the substance that Lavoisier later named oxygen, his work was essential for the abandonment of the long-held theory of phlogiston.

where does most of the oxygen come from on earth The most amount of Oxygen comes from photosynthesis, which uses the energy of sunlight to produce oxygen from water and carbon dioxide. when was it discovered? In the year 1773. by whom? It was discovered independently by Carl Wilhelm Scheele, in Uppsala, in 1773 what are it's uses? Low Pressure Breathing gas in Modern Space Suits, Scuba Divers rely on artificially delivered Oxygen & Medically for Oxygen Masks among many other uses. is it toxic? Can be Toxic at elevated partial pressures, leading to convulsions and other health problems. did the one who discovered it publish any paper regarding this? No how did he die? He contracted a disease of the skin, which, combined with kidney problems caused his death. However, Doctors said he died of Mercury Poisoning when did it happen? On 21 May 1786 at his home in Köping.
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Carl Wilhelm Scheele [SEP] The theory of phlogiston
Bergman informed Scheele that the saltpeter he had purchased from Scheele's employer, after long heating, produced red vapors (now known to be nitrogen dioxide) when it came into contact with acetic acid. Scheele's quick explanation was that the saltpeter had absorbed phlogiston with the heat (had been reduced to nitrite, in modern terms) and gave off a new phlogisticated gas as an active principle when combined with an acid (even a weak acid). Bergman next suggested that Scheele analyze the properties of manganese (IV) oxide. It was through his studies of manganese (IV) oxide that Scheele developed his concept of "fire air" (his name for oxygen).