Reusing the 2019 approach to detecting custom property adoption, the results are: 14.47% desktop and 19.29% mobile. Using the new "css-variables" custom metric, the results are: 27.27% desktop and 26.50% mobile.

custom_props_adoption1.sql

#standardSQL
# 02_01: % of sites that use custom properties.
CREATE TEMPORARY FUNCTION usesCustomProps(css STRING)
RETURNS BOOLEAN LANGUAGE js AS '''
try {
  var reduceValues = (values, rule) => {
    if ('rules' in rule) {
      return rule.rules.reduce(reduceValues, values);
    }
    if (!('declarations' in rule)) {
      return values;
    }

    return values.concat(rule.declarations.filter(d => d.property.startsWith(`--`)));
  };
  var $ = JSON.parse(css);
  return $.stylesheet.rules.reduce(reduceValues, []).length > 0;
} catch (e) {
  return false;
}
''';

SELECT
  client,
  COUNTIF(num_stylesheets > 0) AS freq,
  total,
  ROUND(COUNTIF(num_stylesheets > 0) * 100 / total, 2) AS pct
FROM (
  SELECT
    client,
    page,
    COUNTIF(usesCustomProps(css)) AS num_stylesheets
  FROM
    `httparchive.almanac.parsed_css`
  WHERE
    date = '2020-08-01'
  GROUP BY
    client,
    page)
JOIN
  (SELECT _TABLE_SUFFIX AS client, COUNT(0) AS total FROM `httparchive.summary_pages.2020_08_01_*` GROUP BY client)
USING
  (client)
GROUP BY
  client,
  total

custom_props_adoption2.sql

#standardSQL
# Percent of pages that use custom properties.
CREATE TEMPORARY FUNCTION getCustomPropertyCount(payload STRING) RETURNS INT64 LANGUAGE js AS '''
try {
  var $ = JSON.parse(payload);
  var vars = JSON.parse($['_css-variables']);
  return Object.values(vars.summary).length;
} catch (e) {
  return 0;
}
''';

SELECT
  client,
  COUNTIF(has_custom_properties) AS freq,
  total,
  COUNTIF(has_custom_properties) / total AS pct
FROM (
  SELECT
    _TABLE_SUFFIX AS client,
    getCustomPropertyCount(payload) > 0 AS has_custom_properties
  FROM
    `httparchive.pages.2020_08_01_*`)
JOIN (
  SELECT
    _TABLE_SUFFIX AS client,
    COUNT(DISTINCT url) AS total
  FROM
    `httparchive.pages.2020_08_01_*`
  GROUP BY
    _TABLE_SUFFIX)
USING (client)
GROUP BY
  client,
  total