docs: shared Oasis branding (logo, palette, home link)

Add the Oasis logo (light/dark, always visible in the Furo sidebar), the maroon/red
Raleway palette matching oasislmf.github.io, and a persistent 'Oasis documentation home'
link (rewritten to page-relative by the aggregator) so you can return to the top of the
aggregated site from anywhere.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: sstruzik

docs/source/_static/OASIS_LMF_COLOUR.png

@@ -50,3 +50,28 @@
     _k: (_v[0], _v[1])
     for _k, _v in _ix_json.loads(_ix_os.environ.get("OASIS_INTERSPHINX_MAP", "{}")).items()
 })
+# -- Oasis shared branding (logo, palette, home link) -----------------------
+import os as _os_brand
+if globals().get("html_theme") == "furo":
+    if "_static" not in (globals().get("html_static_path") or []):
+        html_static_path = list(globals().get("html_static_path") or []) + ["_static"]
+    try:
+        html_theme_options
+    except NameError:
+        html_theme_options = {}
+    html_theme_options.setdefault("light_logo", "OASIS_LMF_COLOUR.png")
+    html_theme_options.setdefault("dark_logo", "OASIS_LMF_WHITE.png")
+    _lcv = html_theme_options.setdefault("light_css_variables", {})
+    _lcv.setdefault("color-brand-primary", "#862633")
+    _lcv.setdefault("color-brand-content", "#d22630")
+    _lcv.setdefault("font-stack", "Raleway, sans-serif")
+    _dcv = html_theme_options.setdefault("dark_css_variables", {})
+    _dcv.setdefault("color-brand-primary", "#e2919b")
+    _dcv.setdefault("color-brand-content", "#ef8b93")
+    _home = _os_brand.environ.get("OASIS_DOCS_HOME", "https://oasislmf.github.io/index.html")
+    html_theme_options.setdefault(
+        "announcement",
+        '<a href="' + _home + '" style="color:inherit;font-weight:600;text-decoration:none">'
+        '&#8962; Oasis documentation home</a>')
+    if "https://fonts.googleapis.com/css?family=Raleway" not in (globals().get("html_css_files") or []):
+        html_css_files = list(globals().get("html_css_files") or []) + ["https://fonts.googleapis.com/css?family=Raleway"]