webpack-bootstrap-ui-kit/src/scss/_components/_ui.form.basics.scss

// date-time fields
input.date,
input.time {
    &[readonly] {
        background-color: $white;
    }
}

.bootstrap-timepicker-widget,
.datepicker-dropdown {
    border: 1px solid #ced4da;
    box-shadow: 0 0 3px #999;
}

.bootstrap-timepicker-widget {
    .glyphicon {
        -moz-osx-font-smoothing: grayscale;
        -webkit-font-smoothing: antialiased;
        display: inline-block;
        font-style: normal;
        font-variant: normal;
        text-rendering: auto;
        line-height: 1;
        font-family: Font Awesome\5 Free;
        font-weight: 900;
    }

    .glyphicon-chevron-up:before {
        content: '\f077';
    }

    .glyphicon-chevron-down:before {
        content: '\f078';
    }

    input {
        border: 1px solid #ced4da;
    }
}

.form-inline {
    margin-top: -1rem;

    fieldset {
        margin-top: 1rem;
    }

    .field {
        display: flex;
    }

    .form-control {
        width: 100%;
    }
}

// select2 dropdowns bootstrap4 styling
.select2-container {
    display: block;
    width: 100% !important;
}

.select2-container--default {
    &.select2-container--focus {
        .select2-selection {
            color: $input-focus-color;
            background-color: $input-focus-bg;
            border-color: $input-focus-border-color;
            outline: 0;

            // Avoid using mixin so we can pass custom focus shadow properly
            @if $enable-shadows {
                box-shadow: $input-box-shadow, $input-focus-box-shadow;
            } @else {
                box-shadow: $input-focus-box-shadow;
            }

            &.select2-selection--single {
            }

            &.select2-selection--multiple {
            }
        }
    }

    .select2-selection {
        @extend .form-control;

        .select2-selection__rendered {
            line-height: inherit;
        }

        &.select2-selection--single {
        }

        &.select2-selection--multiple {
            padding-top: 0;
            padding-bottom: 0;

            .select2-selection__choice {
                @extend .badge;

                @extend .badge-primary;

                padding: $input-padding-y $input-padding-x;
                border: 0;
            }

            .select2-selection__choice__remove {
                color: color-yiq($primary);
            }
        }
    }

    .select2-results__option--highlighted[aria-selected] {
        background: $primary;
        color: color-yiq($primary);
    }
}

.select2-dropdown {
    border: $input-border-width solid $input-border-color;

    .select2-search--dropdown {
        padding: $input-padding-y $input-padding-x;
    }

    .select2-search__field {
        @extend .form-control;
    }
}