tl;dr
1) The most significant bit of all characters in the text is 0.
2) Perhaps "the" is one of the most common 3-letter pattern in the text.
3) Every "the" is likely to have a space before and after.
4) Plus a bit of guessing and luck :)

Original writeup (https://github.com/hnn4abo/ctf/tree/master/timisoaractf-2018-quals/b64).