gristlabs_grist-core/app/client/widgets/CheckBox.css

.widget_checkbox {
  position: relative;
  margin: -1px auto;
  width: 16px;
  height: 16px;
}

:not(.formula_field)>.field_clip.has_cursor>.widget_checkbox {
  cursor: pointer;
  box-shadow: inset 0 0 0 1px #606060;
  border-radius: 3px;
  background: linear-gradient(to bottom, rgba(255,255,255,1) 0%, rgba(252,252,252,1) 29%, rgba(239,239,239,1) 50%, rgba(232,232,232,1) 50%, rgba(242,242,242,1) 100%);
}
:not(.formula_field)>.field_clip>.widget_checkbox:hover:not(.disabled) {
  cursor: pointer;
  box-shadow: inset 0 0 0 1px #606060;
  border-radius: 3px;
  background: linear-gradient(to bottom, rgba(255,255,255,1) 0%, rgba(252,252,252,1) 29%, rgba(239,239,239,1) 50%, rgba(232,232,232,1) 50%, rgba(242,242,242,1) 100%);
}

:not(.formula_field)>.field_clip>.widget_checkbox:active:not(.disabled) {
  background: linear-gradient(to bottom, rgba(147,180,242,1) 0%, rgba(135,168,233,1) 10%, rgba(115,149,218,1) 25%, rgba(115,150,224,1) 37%, rgba(115,153,230,1) 50%, rgba(86,134,219,1) 51%, rgba(130,174,235,1) 83%, rgba(151,194,243,1) 100%);
}

.widget_checkbox:focus {
  outline: none !important;
}

.widget_checkmark {
  position: relative;
  width: 6px;
  height: 12px;
  -ms-transform: rotate(40deg); /* IE 9 */
  -webkit-transform: rotate(40deg); /* Chrome, Safari, Opera */
  transform: rotate(40deg);
  left: 4px;
  top: 2px;
}

.checkmark_stem {
  position: relative;
  width: 3px;
  height: 12px;
  background-color: var(--grist-actual-cell-color, #606060);
  border: 1px solid var(--grist-actual-cell-color, #606060);
  left: 3px;
  top: -5px;
}

.checkmark_kick {
  position: relative;
  width: 3px;
  height: 3px;
  background-color: var(--grist-actual-cell-color, #606060);
  border: 1px solid var(--grist-actual-cell-color, #606060);
  top: 7px;
}