Born in Brig-Glis, Switzerland, Theler began his career from the youth team of FC Sion, where he had featured for the clubs' under-21 division from 2016 to 2019.
Sion
2019–20
Theler signed his first senior contract with the Swiss top-tier club FC Sion on 2019. He would represent for the club in the 2019-20 Swiss Super League season. Theler played his debut match for the club against FC Basel on 28 June 2020 as a substitute for Dimitri Cavaré on the 50th minute of the game, which ended 2–0 to Basel. Theler scored his debut goal for the club in his last match of the season against FC Luzern on 22 July 2020 in the 31st minute of the game. The match ended 1–2 to Sion.