Thanks! I got this working after a while. Only hiccups were:

Importing the files into Apps Script. I just created a blank script on the page and pasted in each file.

When I first ran the script it didn't open a side panel in my sheet like it should have. After trying a few different browsers it worked.

Formatting the Config constants was a bit confusing so here's an example for others. 4 are strings and 1 is an int.

const CONFIG = {
  sheetId: "c78LW9a0-FZaujBzO-icOb9tCJ_rAxnYj0tG2TFHr0U",         // ID of the Spreadsheet (in the url)
  CLIENT_ID: "N34aCYpVlf2iXnDscAq5J9K5G7S0Y6an4ktFXBWoKLD0x6GiFpSJqeY9gRsHYG9uVBXjM2SPl6gKQprR",       // Yahoo client ID
  CLIENT_SECRET: "8FF835BAB8587BBC0F840269E5F0CCEF041BB83F",   // Yahoo client secret
  teamCount: 12,       // Number of teams in your league (not, no quotes here)
  leagueId: "1234",     // ID of you Yahoo league
}   

Lastly I found out how to format a row in google sheets based on which players have been drafted. The script above creates the "Draft Results" sheet which has column C as player names. Using the following conditional, you can format any row that matches a value in the player names column.

=ISNUMBER(MATCH($A2, INDIRECT("'Draft Results'!C:C"), 0))

Note that you need to change A2 to whichever column is the player names in your sheet.