frontend update to tabler beta 19 (#4029)

assets/sass/variables.scss

@@ -1,30 +0,0 @@
-/*
- * This file is part of the Kimai time-tracking app.
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-@import "bootstrap/scss/functions";
-@import "bootstrap/scss/variables";
-@import "bootstrap/scss/mixins";
-
-@import "@tabler/core/src/scss/variables";
-@import "@tabler/core/src/scss/mixins";
-
-/* For highlighting rows, fieldsets and so on */
-$highlight-bg: #f5f5f5;
-$highlight-today: #fcf8e3;
-
-$red: #dd4b39;
-$green: #00a65a;
-$yellow: #f0ad4e;
-$blue: #0073b7;
-
-$gray-lte: #d2d6de;
-$kimai-default: $gray-lte;
-
-$box-border-radius: 3px;
-
-$dark-mode-today: var(--tblr-blue);
-$dark-mode-weekend: lighten($dark, 5%) !default;