Hi… I am well aware that this diff view is very suboptimal. It will be fixed when the refactored server comes along!
Add basic stylesheet
body {
	font-family: sans-serif;
	margin: 0;
color: #444;
}
main {
main, #nav-inner {
padding: 0 5px; max-width: 800px; margin: 0 auto;
}
main {
	padding-bottom: 20px;
}
nav {
	border-bottom: 1px solid #eee;
}
nav h1 {
	margin: 0;
	padding: 10px 0;
	font-size: 1.2em;
}
nav h1 a {
	color: inherit;
	text-decoration: none;
}
h2 {
	font-size: 1.2em;
}
table {
	border-collapse: collapse;
}
td, th {
border: 1px solid black;
border: 1px solid rgb(208, 210, 215);
padding: 5px; }
button {
	border: 1px solid rgb(208, 210, 215);
	border-radius: 4px;
	padding: 6px 12px;
	margin: 4px 0;
	color: #444;
	background-color: transparent;
	cursor: pointer;
}
button:hover {
	background-color: rgba(0, 0, 0, 0.02);
}
button[type="submit"]:not(.btn-regular):first-of-type {
	background-color: rgb(0, 128, 0);
	border-color: rgb(0, 128, 0);
	color: white;
}
button[type="submit"]::not(.btn-regular):first-of-type:hover {
	background-color: rgb(0, 150, 0);
	border-color: rgb(0, 150, 0);
}
input[type="text"], input[type="password"], input[type="url"], textarea {
	border: 1px solid rgb(208, 210, 215);
	border-radius: 4px;
	padding: 6px;
	margin: 4px 0;
	color: #444;
}
input[type="text"]:focus, input[type="password"]:focus, input[type="url"]:focus, textarea:focus {
	outline: none;
	border-color: rgb(0, 128, 0);
}
label {
	display: block;
	margin: 15px 0;
}
label input[type="text"], label input[type="password"], label input[type="url"] {
	display: block;
	width: 100%;
	max-width: 350px;
	box-sizing: border-box;
}
label:has(input[type="radio"]) {
	margin: 5px 0;
}
label textarea {
	display: block;
	width: 100%;
	resize: vertical;
}
main.narrow {
	max-width: 400px;
}
main.narrow input {
	max-width: 100%;
}
{{ template "head.html" .Base }}
<main>
<main class="narrow">
<h1>{{ .ServerName }}</h1>
<p>
	Authorize
	{{ if .Client.ClientURI }}
		<a href="{{ .Client.ClientURI }}" target="_blank">
	{{ end }}
	{{- if .Client.ClientName -}}
		{{- .Client.ClientName -}}
	{{- else -}}
		<code>{{- .Client.ClientID -}}</code>
	{{- end -}}
	{{- if .Client.ClientURI -}}
		</a>
	{{- end -}}
	?
</p>
<form method="post" action="">
	<button type="submit" name="authorize">Authorize</button>
	<button type="submit" name="deny">Cancel</button>
</form>
</main>
{{ template "foot.html" }}
{{ template "head.html" .Base }}
{{ template "nav.html" .Base }}
<main>
<h1>{{ .ServerName }}</h1>
<h2>Client credentials</h2>
<p>
	<strong>Client ID</strong>: <code>{{ .ClientID }}</code><br>
	<strong>Client secret</strong>: <code>{{.ClientSecret}}</code>
</p>
<a href="/">Back</a>
<a href="/"><button type="button">Back</button></a>
</main>
{{ template "foot.html" }}
{{ template "head.html" .Base }}
{{ template "nav.html" .Base }}
<main>
<h1>{{ .ServerName }}</h1>
<p>Welcome, {{ .Me.Username }}!</p>
<form method="post">
	<a href="/user/{{ .Me.ID }}"><button type="button">Settings</button></a>
<button type="submit" formaction="/logout">Logout</button>
<button type="submit" formaction="/logout" class="btn-regular">Logout</button>
</form>
<h2>Authorized clients</h2>
{{ with .AuthorizedClients }}
	<table>
		<tr>
			<th>Client</th>
			<th>Authorized until</th>
			<th></th>
		</tr>
		{{ range . }}
			<tr>
				<td>
					{{ with .Client }}
						{{ if .ClientURI }}
							<a href="{{ .ClientURI }}" target="_blank">
						{{ end }}
						{{ if .ClientName }}
							{{ .ClientName }}
						{{ else }}
							<code>{{ .ClientID }}</code>
						{{ end }}
						{{ if .ClientURI }}
							</a>
						{{ end }}
					{{ end }}
				</td>
				<td>{{ .ExpiresAt }}</td>
				<td>
					<form method="post" action="/client/{{ .Client.ID }}/revoke">
						<button type="submit">Revoke</button>
					</form>
				</td>
			</tr>
		{{ end }}
	</table>
{{ else }}
	<p>No client authorized yet.</p>
{{ end }}
{{ if .Me.Admin }}
	<h2>Registered clients</h2>
	{{ with .Clients }}
		<p>
			<a href="/client/new"><button type="button">Register new client</button></a>
		</p>
<p> <a href="/client/new"><button type="button">Register new client</button></a> </p>
	{{ with .Clients }}
		<table>
			<tr>
				<th>Client ID</th>
				<th>Name</th>
			</tr>
			{{ range . }}
				<tr>
					<td><a href="/client/{{ .ID }}"><code>{{ .ClientID }}</code></a></td>
					<td>{{ .ClientName }}</td>
				</tr>
			{{ end }}
		</table>
	{{ else }}
		<p>No client registered yet.</p>
	{{ end }}
	<h2>Users</h2>
	<p>
		<a href="/user/new"><button type="button">Create user</button></a>
	</p>
	<table>
		<tr>
			<th>Username</th>
			<th>Role</th>
		</tr>
		{{ range .Users }}
			<tr>
				<td><a href="/user/{{ .ID }}">{{ .Username }}</a></td>
				<td>
					{{ if .Admin }}
						Administrator
					{{ else }}
						Regular user
					{{ end}}
				</td>
			</tr>
		{{ end }}
	</table>
{{ end }}
</main>
{{ template "foot.html" }}
{{ template "head.html" .Base }}
<main>
<main class="narrow">
<h1>{{ .ServerName }}</h1>
<form method="post" action="">
Username: <input type="text" name="username" autocomplete="username"><br> Password: <input type="password" name="password"><br>
<label> Username <input type="text" name="username" autocomplete="username" autofocus> </label> <label> Password <input type="password" name="password"> </label>
	<button type="submit">Login</button>
</form>
</main>
{{ template "foot.html" }}
{{ template "head.html" .Base }}
{{ template "nav.html" .Base }}
<main>
<h1>{{ .ServerName }}</h1>
<h2>
	{{ if .Client.ID }}
		Update client
	{{ else }}
		Create client
	{{ end }}
</h2>
<form method="post" action="">
	{{ if .Client.ClientID }}
		Client ID: <code>{{ .Client.ClientID }}</code><br>
	{{ end }}
	Name: <input type="text" name="client_name" value="{{ .Client.ClientName }}"><br>
	Website: <input type="url" name="client_uri" value="{{ .Client.ClientURI }}"><br>
	<label>
		Name
		<input type="text" name="client_name" value="{{ .Client.ClientName }}">
	</label>
	<label>
		Website
		<input type="url" name="client_uri" value="{{ .Client.ClientURI }}">
	</label>
Client type:
	{{ if .Client.ID }}
		{{ if .Client.IsPublic }}
			public
		{{ else }}
			confidential
		{{ end }}
		<br>
		<p>
			Client type
			<br>
			<strong>
			{{ if .Client.IsPublic }}
				Public
			{{ else }}
				Confidential
			{{ end }}
			</strong>
		</p>
	{{ else }}
<br>
Client type
<label> <input type="radio" name="client_type" value="confidential" checked> Confidential </label>
<br>
<label> <input type="radio" name="client_type" value="public"> Public </label>
<br>
	{{ end }}
	Redirect URIs:<br>
	<textarea name="redirect_uris">{{ .Client.RedirectURIs }}</textarea><br>
	<small>The special URI <code>http://localhost</code> matches all loopback interfaces.</small><br>
	<label>
		Redirect URIs
		<textarea name="redirect_uris" wrap="off">{{ .Client.RedirectURIs }}</textarea>
		<small>The special URI <code>http://localhost</code> matches all loopback interfaces.</small><br>
	</label>
	<button type="submit">
		{{ if .Client.ID }}
			Update client
		{{ else }}
			Create client
		{{ end }}
	</button>
	{{ if .Client.ID }}
		{{ if not .Client.IsPublic }}
			<button type="submit" name="rotate">Rotate client secret</button>
		{{ end }}
		<button type="submit" name="delete">Delete client</button>
	{{ end }}
	<a href="/"><button type="button">Cancel</button></a>
</form>
</main>
{{ template "foot.html" }}
{{ template "head.html" .Base }}
{{ template "nav.html" .Base }}
<main>
<h1>{{ .ServerName }}</h1>
<h2>
	{{ if .User.ID }}
		Update user
	{{ else }}
		Create user
	{{ end }}
</h2>
<form method="post" action="">
	Username: <input type="text" name="username" value="{{ .User.Username }}" required><br>
	Password: <input type="password" name="password"><br>
	<label>
		Username
		<input type="text" name="username" value="{{ .User.Username }}" required>
	</label>
	<label>
		Password
		<input type="password" name="password">
	</label>
	{{ if not (eq .Me.ID .User.ID) }}
		<label>
			<input type="checkbox" name="admin" {{ if .User.Admin }}checked{{ end }}>
			Administrator
</label><br>
</label>
	{{ end }}
	<button type="submit">
		{{ if .User.ID }}
			Update user
		{{ else }}
			Create user
		{{ end }}
	</button>
	<a href="/"><button type="button">Cancel</button></a>
</form>
</main>
{{ template "foot.html" }}
<nav>
	<div id="nav-inner">
		<h1><a href="/">{{ .ServerName }}</a></h1>
	</div>
</nav>