dialog.css

/**
 * @file
 * jQuery UI dialog styles.
 */
.ui-widget {
  font-size: inherit;
  font-family: inherit;
}

.ui-dialog {
  border: 0.125em solid #20252e;
  -webkit-backface-visibility: hidden;
  backface-visibility: hidden;
}

.ui-corner-all {
  border-radius: 0;
}

.ui-dialog .ui-dialog-titlebar {
  cursor: default;
  -webkit-touch-callout: none;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

.ui-dialog .ui-dialog-titlebar-close {
  top: 0;
  top: calc(-0.125em);
  right: 0;
  right: calc(-0.125em);
  width: 2.5em;
  height: 100%;
  height: calc(100% + 0.125em);
  margin: 0;
  border: 0;
  outline: none;
}

.ui-dialog .ui-dialog-titlebar-close .ui-icon {
  top: 50%;
  left: 50%;
  -webkit-transform: translate(-50%, -50%);
  transform: translate(-50%, -50%);
  width: 1em;
  height: 1em;
  margin: 0;
  background: transparent;
}

/**
 * Close X CSS art
 */
.ui-dialog .ui-dialog-titlebar-close .ui-icon:before,
.ui-dialog .ui-dialog-titlebar-close .ui-icon:after {
  content: '';
  position: absolute;
  top: 0;
  left: 0;
  display: block;
  width: 0.125em;
  height: 0.875em;
}

.ui-dialog .ui-dialog-titlebar-close .ui-icon:before {
  -webkit-transform:
    translate(0.5em, 0.125em)
    rotate(45deg);
  transform:
    translate(0.5em, 0.125em)
    rotate(45deg);
}

.ui-dialog .ui-dialog-titlebar-close .ui-icon:after {
  -webkit-transform:
    translate(0.5em, 0.125em)
    rotate(-45deg);
  transform:
    translate(0.5em, 0.125em)
    rotate(-45deg);
}

.ui-button-icon-only .ui-button-text,
.ui-button-icons-only .ui-button-text {
  padding: 0;
}