feat: support custom css theme via environment variable

pywebio/platform/tpl/index.html

@@ -8,7 +8,11 @@
     <link rel="icon" type="image/png" sizes="32x32" href="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAYAAAAf8/9hAAAAwklEQVQ4T63TvU5CQRCG4WcwMfEuqOgNtQ2Nd4CxV2LHtVhJ0N7AHdjQUBtrrLwLA4ks2Rx+/Qucw3Y78807M7sz4ft5dq6mI7RQX7o/JCNzfdfetkNifRk6k9wLN9jYdxMkyZPQ1faZXYUwB/OCix8V/W4Y4zJDCsBAX7jdM7iQJY+udELu+cTrP2X/xU2+NMPAg3B3UPaVOOmFoQkapQC8Z8AUpyUBs6MAKrZQ+RErf2PlQTrKKK8gpZdpewgOXOcFTTxEjYwMoIkAAAAASUVORK5CYII=" id="favicon32">
     <link rel="icon" type="image/png" sizes="16x16" href="data:image/png;base64,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" id="favicon16">
     <link rel="stylesheet" href="{{ base_url }}css/markdown.min.css">
+    {% if bootstrap_css %}
+    <link rel="stylesheet" href="{{ bootstrap_css }}">
+    {% else %}
     <link rel="stylesheet" href="{{ base_url }}css/bootstrap.min.css">
+    {% end %}
     <link rel="stylesheet" href="{{ base_url }}css/codemirror.min.css">
     <link rel="stylesheet" href="{{ base_url }}codemirror/base16-light.min.css">
     <link rel="stylesheet" href="{{ base_url }}css/toastify.min.css">

pywebio/platform/utils.py

@@ -5,7 +5,7 @@ from collections import defaultdict
 from collections import namedtuple
 from collections.abc import Mapping, Sequence
 from functools import partial
-from os import path
+from os import path, environ
 
 from tornado import template
 
@@ -23,6 +23,8 @@ MAX_PAYLOAD_SIZE = 0
 
 DEFAULT_CDN = "https://cdn.jsdelivr.net/gh/wang0618/PyWebIO-assets@v{version}/"
 
+BOOTSTRAP_VERSION = '4.4.1'
+
 AppMeta = namedtuple('App', 'title description')
 
 _here_dir = path.dirname(path.abspath(__file__))
@@ -46,9 +48,27 @@ def render_page(app, protocol, cdn):
     else:  # user custom cdn
         cdn = cdn.rstrip('/') + '/'
 
+    bootstrap_css = bootstrap_css_url()
+
     return _index_page_tpl.generate(title=meta.title or 'PyWebIO Application',
                                     description=meta.description, protocol=protocol,
-                                    script=True, content='', base_url=cdn)
+                                    script=True, content='', base_url=cdn, bootstrap_css=bootstrap_css)
+
+
+def bootstrap_css_url():
+    """Get bootstrap theme css url from environment variable PYWEBIO_THEME
+
+    PYWEBIO_THEME can be one of bootswatch themes, or a custom css url. 
+    """
+    theme_name = environ.get('PYWEBIO_THEME')
+    bootswatch_themes = {'flatly', 'yeti', 'cerulean', 'pulse', 'journal', 'cosmo', 'sandstone', 'simplex', 'minty',
+                         'slate', 'superhero', 'lumen', 'spacelab', 'materia', 'litera', 'sketchy', 'cyborg', 'solar',
+                         'lux', 'united', 'darkly'}
+    if theme_name in bootswatch_themes:
+        return 'https://cdn.jsdelivr.net/npm/bootswatch@{version}/dist/{theme}/bootstrap.min.css'.format(
+            version=BOOTSTRAP_VERSION, theme=theme_name)
+
+    return theme_name  # it's a url
 
 
 def cdn_validation(cdn, level='warn', stacklevel=3):