[Client] Add CSS stylesheets for the startup page

Two CSS stylesheets have been added: one which is common to all pages and one
which is specific to the game statup.

client/res/ui/game_ui.css

@@ -0,0 +1,79 @@
+/* Custom fonts */
+@font-face {
+    font-family: 'Spicy Rice';
+    font-style: normal;
+    font-weight: 400;
+    font-display: swap;
+    src: url("../fonts/spicy_rice_v21/spicy_rice_v21.woff2") format("woff2"), url("../fonts/spicy_rice_v21/spicy_rice_v21.woff") format("woff"), url("../fonts/spicy_rice_v21/spicy_rice_v21.ttf") format("truetype");
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+
+@font-face {
+    font-family: 'Titan One';
+    font-style: normal;
+    font-weight: 400;
+    src: url("../fonts/titan_one_v13/titan_one_v13.woff2") format('woff2'), url("../fonts/titan_one_v13/titan_one_v13.woff") format('woff'), url("../fonts/titan_one_v13/titan_one_v13.ttf") format('truetype');
+    unicode-range: U+0000-00FF, U+0131, U+0152-0153, U+02BB-02BC, U+02C6, U+02DA, U+02DC, U+2000-206F, U+2074, U+20AC, U+2122, U+2191, U+2193, U+2212, U+2215, U+FEFF, U+FFFD;
+}
+
+/* Base elements */
+:root {
+    color-scheme: dark;
+    --footer-links-height: 2em;
+}
+  
+html {
+    margin: 0;
+}
+
+body {
+    --body-margin: 0.375em;
+    margin: var(--body-margin);
+}
+
+/* Themes */
+.dark {
+    background-color: #213C40;
+}
+
+.light {
+    background-color: #B1EDE8;
+}
+
+/* Utility classes */
+.hidden {
+    display: none;
+}
+
+/* Footer */
+.footer_link {
+    color: white;
+    font-family: sans-serif;
+    font-size: 1em;
+    font-weight: bold;
+    margin: 0.5em;
+    text-decoration: none;
+}
+
+.footer_link:visited {
+    color: unset;
+}
+
+.footer_links {
+    align-content: center;
+    align-items: center;
+    display: flex;
+    flex-direction: row;
+    flex-wrap: wrap;
+    height: var(--footer-links-height);
+    justify-content: flex-end;
+}
+
+/* Unsupported browser message */
+.unsupported_browser {
+    display: none;
+}
+
+.unsupported_show {
+    display: block;
+}

client/res/ui/start/game_ui_start.css

@@ -0,0 +1,72 @@
+:root {
+    --game-begin-margin: 2em;
+    --header-actions-height: 3em;
+}
+
+.game_begin {
+    align-items: center;
+    border-radius: 1.5em;
+    background-image: url("../../images/start_background.png");
+    background-position: center;
+    background-repeat: no-repeat;
+    background-size: cover;
+    display: flex;
+    flex-direction: column;
+    flex-wrap: wrap;
+    justify-content: center;
+    margin: var(--game-begin-margin);
+    /*
+    Viewport height - body margin (x2: top + bottom) - header actions height
+    - footer links height - game begin margin
+    */
+    height: calc(100vh - var(--body-margin) * 2
+        - var(--game-begin-margin) * 2
+        - var(--header-actions-height)
+        - var(--footer-links-height));
+}
+
+.game_title {
+    color: #BD1E1E;
+    font-family: "Spicy Rice", serif;
+    font-weight: bold;
+    font-size: 5em;
+    margin-block-start: 0;
+    margin-block-end: 0;
+    margin-top: 0.5em;
+    margin-bottom: 0.5em;
+    text-align: center;
+    -webkit-text-stroke: 1px black;
+    text-shadow: 1px 0 0 red, 0 1px 0 red, -1px 0 0 red, 0 -1px 0 red;
+}
+
+.top_button {
+    background-color: transparent;
+    border: none;
+    fill: white;
+    cursor: pointer;
+    padding: 0;
+}
+
+.header_actions {
+    display: flex;
+    justify-content: flex-end;
+    height: var(--header-actions-height);
+}
+
+.action_button {
+    border-color: black;
+    border-style: solid;
+    border-width: 0.125em;
+    background-color: #BD1E1E;
+    border-radius: 1em;
+    cursor: pointer;
+    font-family: "Titan One", sans-serif;
+    font-size: 3em;
+    margin-left: auto;
+    margin-right: auto;
+    padding-top: 0.5em;
+    padding-bottom: 0.5em;
+    padding-left: 1em;
+    padding-right: 1em;
+    text-transform: uppercase;
+}