Here is a complete example of the pelicanconf.py file, as used for the present documentation of the theme. It starts by setting some standard pelican variables, as usual. Then, come the theme's and the optional plugins.

# =============================
# 1. standard pelican variables
# =============================
SITEURL      = 'http://lepagito.artliba.org'
SITENAME     = 'Lepagito'
AUTHOR       = 'I. Lepage and P. Hesse'
SITESUBTITLE = 'A lovely pelican theme'
TIMEZONE     = 'Europe/Paris'
DEFAULT_LANG = 'en'
LOCALE       = 'C'

PATH         = 'content'
STATIC_PATHS = []

OUTPUT_PATH  = 'output'
#DELETE_OUTPUT_DIRECTORY = True # before site generation

DISPLAY_CATEGORIES_ON_MENU = False # choose a custom top menu here
MENUITEMS = (
  ('All',        'index.html'),
  ('Quickstart', 'quickstart.html'),
  ('Goodies',    'goodies.html'),
  ('Options',    'options.html'),
  ('#Tags',      'tags.html'),
)
DEFAULT_PAGINATION =   6; # article summaries per page
SUMMARY_MAX_LENGTH = 100; # counted in words

RELATIVE_URLS                    = True
SLUGIFY_SOURCE                   = 'basename' 
ARTICLE_URL   = ARTICLE_SAVE_AS  = '{slug}.html'
CATEGORY_URL  = CATEGORY_SAVE_AS = '{slug}.html'
TAG_URL       = TAG_SAVE_AS      = '{slug}.html'
AUTHOR_URL    = AUTHOR_SAVE_AS   = '{slug}.html'
ARCHIVES_SAVE_AS                 = 'archives.html'
YEAR_ARCHIVE_SAVE_AS             = 'archive_{date:%Y}.html'
MONTH_ARCHIVE_SAVE_AS            = 'archive_{date:%Y}_{date:%m}.html'
# =============================
# 2. Lepagito theme
# =============================
THEME = '/home/hesse/dvt-perso/lepagito'
FOOTERMENUITEMS = (
  ('About',    'about.html'),
  ('License',  'license.html'),
)
TAGS_BY_GROUPS = (
  ('Pelican', ('tag-group', 'navigation', 'plugin', 'pelicanconf.py')),
)
#FOOTERMENUFEED    = False; # default is True
#ARCHIVES_CALENDAR = False; # default is True
LEPAGITO_NAVIGATION_SWAP = True
LEPAGITO_COLOR = { 'cold': 0, 'file': 'color.css' } # the default value
# internationalization (i18n): here are the default values
I18N_CATEGORIES  = 'Categories'
I18N_CATEGORY    = 'Category'
I18N_TAGS        = '#Tags'
I18N_TOP         = 'Top'
I18N_PAGE        = 'Page'
I18N_READ_MORE   = 'Read more'
I18N_ARCHIVES    = 'Archives'
I18N_FEED_ALL_ATOM = 'Feed'
I18N_NEWEST      = 'Newest'
I18N_OLDEST      = 'Oldest'
I18N_FIRST       = 'First'
I18N_LAST        = 'Last'
I18N_BACK_TO     = 'Back to'
I18N_OTHERS      = 'Others'
I18N_PREV        = 'Prev'
I18N_NEXT        = 'Next'
# =============================
# 3. plugins, optionals
# =============================
PLUGIN_PATHS  = ['/home/hesse/dvt-perso/galleria/pelican/plugins', 
                 '/home/hesse/dvt-perso/theme-config/pelican/plugins',
                 '/home/hesse/pelican-plugins/search/pelican/plugins']
PLUGINS = ['search', 'sitemap', 'galleria', 'neighbors', 'theme_config']
# ------------------
# 3.1. search plugin
# ------------------
LEPAGITO_HAVE_SEARCH_MENU = ('search' in PLUGINS) # activate search menu
# -------------------
# 3.2. sitemap plugin
# -------------------
SITEMAP = {
    'format': 'xml',
    'priorities': {
        'articles': 1.0,
        'pages':    0.5,
        'indexes':  0.5
    },
    'changefreqs': {
        'articles': 'weekly',
        'pages':    'monthly',
        'indexes':  'monthly'
    }
}
# --------------------
# 3.3. galleria plugin
# --------------------
##THEME_TEMPLATES_OVERRIDES      = ['/home/hesse/dvt-perso/galleria/template']
GALLERIA_LIBRARY_PATH          = ['/home/hesse/dvt-perso/lepagito/doc/content']
LEPAGITO_GALLERY_CSS_FILE      = 'galleria.css'
# for lepagito templates:
GALLERIA_PROCESS_DIR           = 'galleria'
GALLERIA_RESIZE_JOBS           = 10
GALLERIA_VERBOSE               = True
GALLERIA_PROCESS = {
    'image-icon': {
        'type': 'image',
        'ops': ['50x50', '+profile "*"'],
    },
    'image-large': {
        'type': 'image',
        'ops': ['1200x900', '+profile "*"'],
    },
    # https://developer.mozilla.org/en-US/docs/Learn/HTML/Multimedia_and_embedding/Responsive_images
    'responsive-small': {
        'type': 'responsive-image',
        'sizes': (
            '(min-width: 768px) 650px, '
            '(min-width: 1200px) 800px, '
            '100vw'
        ),
        'srcset': [
            ('600w', ['600x450', '+profile "*"']),
            ('800w', ['800x600', '+profile "*"']),
        ],
        'default': '800w',
    },
    # https://developer.mozilla.org/en-US/docs/Web/HTML/Element/picture
   'picture-tag': {
       'type': 'picture',
       'sources': [
           {
               'name': 'default',
               'media': '(min-height: 640px)',
               'srcset': [
                   ( '640w', [ '640x480',  '+profile "*"']),
                   ('1024w', ['1024x683',  '+profile "*"']),
                   ('1600w', ['1600x1200', '+profile "*"']),
               ],
               'sizes': '100vw',
           },
           {
               'name': 'source-1',
               'srcset': [
                   ('1x', ['600x450']),
                   ('2x', ['800x600']),
               ]
           },
       ],
       'default': ('default', '640w'),
   },
}
GALLERIA_DEFAULT_CLASS         = 'responsive-small'
GALLERIA_SINGLE_CLASS          = 'image-large'
GALLERIA_ICON_CLASS            = 'image-icon'
GALLERIA_DEFAULT_MAX_HEIGHT    = '600px'

Remark in the first that all URLs are chosen as relative here:

RELATIVE_URLS = True
ARTICLE_URL   = ARTICLE_SAVE_AS  = '{slug}.html'
CATEGORY_URL  = CATEGORY_SAVE_AS = '{slug}.html'

Also, all output files (articles, categories, tags, ...) are written in the same "output/" directory, without any hierarchy. This setting is convenient, since it allows to develop and fully test the blog off-line before to upload it on its final destination: relative URLs, such as in the MENUITEMS, work both in off-line and in-line modes.

The second part of the configuration file contains the specific settings of the Lepagito theme: these settings are be described in the sections of the Goodies category. Finally, the third part of the pelicanconf.py file sets the optional plugins: see the Options category.