hacks-guide-minimal-mistakes-theme/_sass/minimal-mistakes/_tables.scss

/* ==========================================================================
   TABLES
   ========================================================================== */

  table {
  display: table; /* hacks-guide change */
  margin-bottom: 1em;
  width: 100%;
  font-family: $global-font-family;
  font-size: $type-size-6;
  border-collapse: collapse;
  border: 1px solid $light-gray; /* hacks-guide change */
  overflow-x: auto;

  & + table {
    margin-top: 1em;
  }
}

thead {
  background-color: $border-color;
/* hacks-guide change start */
  // border-bottom: 2px solid mix(#000, $border-color, 25%);
  border-bottom: 1px solid $light-gray;
/* hacks-guide change end */
}

th {
  padding: 0.5em;
  font-weight: bold;
  text-align: left;
  border-right: 1px solid $light-gray; /* hacks-guide change */
}

td {
  padding: 0.5em;
/* hacks-guide change start */
  // border-bottom: 1px solid mix(#000, $border-color, 25%);
  border-bottom: 1px solid $light-gray;
  border-right: 1px solid $light-gray;
/* hacks-guide change end */
}

tr,
td,
th {
  vertical-align: middle;
}