You have the url for the demo page href="https://bootswatch.com/solar/"
not the css file.
The bootswatch help page says:
You can also use the themes via CDN at jsDelivr.
So it looks like you use the following for the latest version of the solar theme:
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/solar/bootstrap.css">
Also the javascript should be after your {% block body %}
and you should use only bootstrap.bundle.min.js (single file) OR both popper.min.js and bootstrap.min.js (two files) not all three. I recommend using v4.6.0 to match the bootswatch theme version, which still requires jQuery:
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/jquery.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js"></script>
Here is a working snippet:
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/solar/bootstrap.css">
<h1>test</h1>
<button type="button" class="btn btn-primary">Primary</button>
<button type="button" class="btn btn-secondary">Secondary</button>
<button type="button" class="btn btn-success">Success</button>
<button type="button" class="btn btn-info">Info</button>
<button type="button" class="btn btn-warning">Warning</button>
<button type="button" class="btn btn-danger">Danger</button>
<button type="button" class="btn btn-link">Link</button>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/jquery.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/bootstrap.bundle.min.js"></script>
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