Is sharing an account with a colleague to study a free course on an educational platform considered theft, knowing that the platform may have dedicated the courses to one person?

It depends on the platform's terms and conditions. If it does not prohibit it, then there is no harm. However, if it does prohibit it, then its condition must be adhered to in order to respect intellectual property rights. Personal use is permissible when needed, provided that it is not a means of earning money or engaging in trade.