nixos/services: lay ground for gitea branding

also makes adds gitea persistence

assets/gitea/templates/home.tmpl

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+{{template "base/head" .}}
+<div role="main" aria-label="{{if .IsSigned}}{{ctx.Locale.Tr "dashboard"}}{{else}}{{ctx.Locale.Tr "home"}}{{end}}"
+   class="page-content home tw-mb-8 tw-px-8 center" style="height: 100%; flex-direction: column; display: flex; align-items: center; justify-content: space-around;">
+   <img width="500" height="500" src="{{AssetUrlPrefix}}/images/full-logo.svg" alt="{{ctx.Locale.Tr "logo"}}">
+   <div class="hero">
+      <h1 class="ui icon header title">
+         {{AppName}}
+      </h1>
+      <!-- <h2>My own gitea server 😎</h2> -->
+   </div>
+</div>
+{{template "base/footer" .}}

assets/gitea/theme.css

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+@import "/assets/css/theme-gitea-dark.css";
+
+:root {
+    --is-dark-theme: true;
+
+    --accent-color: 221, 85, 85; /* #d55 */
+    --gitea-color-primary-dark-4: 221, 85, 85;
+    --accent-color-secondary: 96, 72, 10;
+    --accent-color-hover: 170, 68, 68;
+    --color-primary: rgb(var(--accent-color));
+    --color-secondary: rgb(var(--accent-color-secondary));
+    --button-color: rgb(var(--accent-color));
+    --button-color-hover: rgb(var(--accent-color-hover));
+}
+
+.navbar-left > #navbar-logo.item, .navbar-right > #navbar-logo.item, .navbar-mobile-right > #navbar-logo.item {
+    padding: 3px;
+}