This is the documentation on all the theme override functions in Discogs 6.x.

• After you find releases using the import form, the resulting import form is rendered using theme_discogs_forminfo($info). The $info variable is a multidimensional array of artist/label info.
• For releases with multiple artists, you format the "artist" field using theme_discogs_artists($artists, $cleanup = TRUE). The $artists variable is a multidimensional array artist names and join characters. $cleanup is a boolean variable that says whether you should "clean up" artists' names; that is, convert "Artist, the (2)" into "Artist." The discogs code uses a helper function to cleanup the artist name, which is called _discogs_cleanup_artist_field($artist).
• For releases with multiple labels, you format the label" field using theme_discogs_labels($labels).
• The custom theme function for formatting the "track_notes" field, is called theme_discogs_extra_artists($extraartists, $cleanup = TRUE). This is the one you want to use for release credits. It works much the same as theme_discogs_artists().

  Note: The array that is passed to this function is generated automatically from the JSON returned by Discogs.com. That means that the track info is in the array - it's just not displayed. It also means that you can use some array-merging trickery to display it however you want.

• To theme the "format" field, you use theme_discogs_formats($formats). It works similar to theme_discogs_labels().
• Finally, there is theme_discogs_checkbox_text($release). This is to format the text that will be beside a checkbox, in the form where you import releases. The $release variable is a rather complicated array, so I'll just let the code do the talking there.

The code for all of these functions is below. To override it, make a function with the same signature, but substitute "yourthemename" (whatever that is) instead of "theme".

<?php
/**
 * Custom theme function for formatting artist/label information on Discogs.com
 * import form.
 *
 * @param $info_array
 *   Multidimensional array of artist/label info
 * @return
 *   HTML markup of information
 * @see _discogs_cleanup_artist_field()
 */
function theme_discogs_forminfo($info) {
  $output = '<div class="discogs-form-info">';
  if (isset($info['images']['image']['uri150'])) {
    $output .= theme_image($info['images']['image']['uri150'], '', '', NULL, FALSE);
  }
  elseif (isset($info['images']['image'][0]['uri150'])) {
    $output .= theme_image($info['images']['image'][0]['uri150'], '', '', NULL, FALSE);
  }
  if (is_array($info['urls']['url']) && isset($info['name'])) {
    $output .= "\n<h3>" . l($info['name'], $info['urls']['url'][0]) . "</h3>";
  }
  elseif (isset($info['urls']['url']) && isset($info['name'])) {
    $output .= "\n<h3>" . l($info['name'], $info['urls']['url']) . "</h3>";
  }
  elseif (isset($info['name'])) {
    $output .= '\n<h3>' . check_plain($info['name']) . '</h3>';
  }
  if (isset($info['namevariations']['name'])) {
    $out = (is_array($info['namevariations']['name'])
      ? implode(", ", $info['namevariations']['name'])
      : $info['namevariations']['name']);
    $output .= '<p>Name Variations: ' . check_plain($out) . '</p>';
  }
  if (isset($info['aliases']['name'])) {
    $out = (is_array($info['aliases']['name'])
      ? implode(", ", $info['aliases']['name'])
      : $info['aliases']['name']);
    $output .= '<p>Alias: ' . check_plain($out) . '</p>';
  }
  if (isset($info['contactinfo'])) {
    $out = str_replace("\n", "\n<br />", $info['contactinfo']);
    $output .= '<p>Contact: ' . check_markup($out) . '</p>';
  }
  if (isset($info['profile'])) {
    $out = str_replace(array('[a=', ']'), '', $info['profile']);
    $out = str_replace("\n", "\n<br />", $out);
    $output .= '<p>' . check_markup($out) . '</p>';
  }
  if (isset($info['members']['name'])) {
    $output .= '<p>Members: ' . implode(", ", $info['members']['name']) . '</p>';
  }
  $output .= '</div>';
  return $output;
}

/**
 * Custom theme function for formatting releases from Discogs that have more
 * than one primary artist.
 *
 * @param $artists
 *   Multidimensional array of artist names and join characters
 * @param $cleanup
 *   Boolean whether to clean up artist field (default TRUE)
 * @return
 *   String with multiple artists, joined with join characters
 * @see _discogs_cleanup_artist_field()
 */
function theme_discogs_artists($artists, $cleanup = TRUE) {
  $artist_string = '';
  if (isset($artists['name'])) {
    $artist_string = $cleanup
      ? _discogs_cleanup_artist_field($artists['name'])
      : $artists['name'];
  }
  elseif (isset($artists[0]['name'])) {
    foreach ($artists as $artist) {
      $name = $cleanup
        ? _discogs_cleanup_artist_field($artist['name'])
        : $artist['name'];
      $join = empty($artist['join']) ? ' / ' : " {$artist['join']} ";
      $artist_string .= $name . $join;
    }
  }
  return rtrim($artist_string, $join);
}

/**
 * Custom theme function for formatting releases from Discogs that have more
 * than one label and catalog number.
 *
 * @param $labels
 *   Multidimensional array of artist names and join characters
 * @return
 *   String with multiple artists, joined with join characters
 * @see _discogs_cleanup_artist_field()
 */
function theme_discogs_labels($labels) {
  if (isset($labels['name'])) {
    $new_labels['discog_label']   = $labels['name'];
    $new_labels['discog_cat_num'] = $labels['catno'];
  }
  elseif (isset($labels[0]['name'])) {
    $name  = '';
    $catno = '';
    foreach ($labels as $label) {
      $name  .= $label['name']  . ', ';
      $catno .= $label['catno'] . ', ';
    }
    $new_labels['discog_label']   = rtrim($name,  ', ');
    $new_labels['discog_cat_num'] = rtrim($catno, ', ');
  }
  return $new_labels;
}

/**
 * Custom theme function for formatting Discogs 'extraartist' array of artist
 * names and roles.
 *
 * @param $extraartists
 *   Array of artist names/roles
 * @param $cleanup
 *   Boolean whether to clean up artist field (default TRUE)
 * @return
 *   String with themed artists and roles
 * @see _discogs_cleanup_artist_field()
 */
function theme_discogs_extra_artists($extraartists, $cleanup = TRUE) {
  // Convert $extraartists['name'] and $extraartists['role'] to $notes
  $notes = '';
  if (is_array($extraartists) && !empty($extraartists)) {
    foreach ($extraartists as $extra) {
      // $extraartists is sometimes multidimensional and sometimes not
      if (isset($extra['name'])) {
        $name = $cleanup ?
          _discogs_cleanup_artist_field($extra['name']) :
          $extra['name'];
        $notes .= $extra['role'] . ' - <b>' . $name . "</b>\n";
      }
      else {
        // Handle multidimensional array through recursion
        $notes .= theme_discogs_extra_artists($extra, $cleanup);
      }
    }
  }
  return $notes;
}

/**
 * Themes an array of formats, which may or may not be multidimensional.
 *
 * @param $formats
 *   Array (possibly multidimensional) of format names from Discogs
 * @return
 *   String with format names, comma-separated
 */
function theme_discogs_formats($formats) {
  $format_string = '';
  if (isset($formats['name'])) {
    $format_string = $formats['name'];
  }
  elseif (isset($formats[0]['name'])) {
    foreach ($formats as $format) {
      $format_string .= $format['name'] . ', ';
    }
  }
  return rtrim($format_string, ', ');
}

/**
 * Takes an array of release information, and outputs text that can be
 * displayed in a checkbox form element.
 *
 * @param $release
 *   Release array. Structure:
 * @code
 *   [release] => array
 *   (
 *     [id] => (Discogs ID)
 *     [status] => (usually "Accepted")
 *     [type] => (usually "Main," sometimes "TrackAppearance")
 *     [title] => (Album title)
 *     [format] => (Album format)
 *     [label] => (Record label)
 *     [year] => (4-digit Year, if any)
 *     [artist] => (Artist, if any)
 *     [summary] => (Summary, if any)
 *   )
 * @endcode
 * @return
 *   Text to put into checkbox fields
 * @see discogs_import_step2()
 */
function theme_discogs_checkbox_text($release) {
  $artist = isset($release['artist'])
    ? check_plain($release['artist'])
    : theme('discogs_artists', $release['artists']['artist']);
  $text  = empty($artist) ? '' : "$artist - ";
  $text .= "<b>{$release['title']}</b>";
  $text .= empty($release['format']) ? '' : " {$release['format']}";
  $text .= empty($release['year']) ? '' : " ({$release['year']})";
  return $text;
}
?>

Additionally, there is an entire template file dedicated to displaying releases, and a CSS file. These are discogs-releast.tpl.php and discogs.css respectively. I won't post the code for these here.

Hope this helps!