[Feat] Improve background color and loading spinner on loading layout

vizro-core/changelog.d/20240725_105946_maximilian_schulz_improved_load_CSS.md

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+<!--
+A new scriv changelog fragment.
+
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+-->
+
+<!--
+### Highlights ✨
+
+- A bullet item for the Highlights ✨ category with a link to the relevant PR at the end of your entry, e.g. Enable feature XXX ([#1](https://github.com/mckinsey/vizro/pull/1))
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+-->
+
+### Added
+
+- Add dark mode and loading spinner to the layout loading screen (before Vizro app is shown) ([#598](https://github.com/mckinsey/vizro/pull/598))
+
+<!--
+### Changed
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+### Security
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vizro-core/src/vizro/static/css/loading.css

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+/* Inspired by https://github.com/facultyai/dash-bootstrap-components/blob/5c8f4b40f1100fc00bf2d5c1d671a7815a6b2910/docs/static/loading.css */
+
+/* This creates a dark background in situations where neither dash-loading nor the Vizro app are displayed */
+html {
+  background: rgba(20, 23, 33, 1);
+  min-height: 100vh;
+}
+
+/* The dash-loading Div is present when Dash is initially loading, before the layout is built */
+
+/* The dash-loading-callback Div is present when Dash has loaded, but the layout is still building */
+
+/* Note that the dash-loading-callback Div is present until all elements are loaded, but as soon as the initial page
+elements (before on-page-load) are rendered, it gets pushed outside the viewable area, hence the spinner is not visible
+which is good, as we have individual loading spinners for elements.
+ */
+._dash-loading-callback,
+._dash-loading {
+  align-items: center;
+  background: rgba(20, 23, 33, 1);
+  color: transparent;
+  display: flex;
+  height: 100%;
+  justify-content: center;
+  position: fixed;
+  width: 100%;
+}
+
+/* Loading spinner */
+._dash-loading-callback::after,
+._dash-loading::after {
+  animation: spinner-border 0.75s linear infinite;
+  border: 0.5rem solid lightgrey;
+  border-radius: 50%;
+  border-right-color: transparent;
+  content: "";
+  display: inline-block;
+  height: 8rem;
+  width: 8rem;
+}