Rambox-CE/ext/packages/ext-theme-base/sass/src/selection/CheckboxModel.scss

.#{$prefix}column-header-checkbox .#{$prefix}column-header-text {
    display: block;
    background-repeat: no-repeat;
    font-size: 0;
}

.#{$prefix}grid-cell-row-checker {
    vertical-align: middle;
    background-repeat: no-repeat;
    font-size: 0;
}