diff --git a/bin/omarchy-theme-install b/bin/omarchy-theme-install
index fb5acdc2..6b442678 100755
--- a/bin/omarchy-theme-install
+++ b/bin/omarchy-theme-install
@@ -37,7 +37,7 @@ THEME_PATH="$THEMES_DIR/$THEME_NAME"
 # ~/.config/omarchy with it, and one called `a';'id` would carry its own
 # command into that picker. The leading character is kept out of `.` and `-`,
 # which also covers `host:-s/foo.git` leaving basename with `.git`.
-if [[ ! $THEME_NAME =~ ^[a-z0-9][a-z0-9._-]*$ ]]; then
+if [[ ! $THEME_NAME =~ ^[a-z0-9_][a-z0-9._+-]*$ ]]; then
   echo "Error: '$REPO_URL' does not give a usable theme name."
   exit 1
 fi
diff --git a/manual/43-making-your-own-theme.md b/manual/43-making-your-own-theme.md
index ed6aee82..c6818a4b 100644
--- a/manual/43-making-your-own-theme.md
+++ b/manual/43-making-your-own-theme.md
@@ -38,6 +38,8 @@ There's a fully commented `alacritty.toml.tpl.sample` in that folder to copy fro
 
 If you want to distribute your theme so others can use it, you need to put it on a public git server, like GitHub. Then people can install it using _Install > Style > Theme_ in the Omarchy menu using that URL. It's recommended that you follow the naming convention of `omarchy-[themename]-theme`, as the theme will show correctly as just `[themename]` in the theme selection menu after installation.
 
+That leftover `[themename]` becomes the theme's directory name, so it has to be one Omarchy can hand around safely: it must start with a letter, a digit, or an underscore, and the rest may hold letters, digits, `.`, `_`, `+`, and `-`. Capitals are lowercased for you, but anything else — a space, a quote, a non-English character — is refused at install time rather than turned into a directory name. So `omarchy-tokyo-night-theme`, `omarchy-flexoki_light-theme`, and `omarchy-c++-theme` all install fine.
+
 Remember that once it's installed from a repo, any `.lua`, terminal config or `vscode.json` it ships is dropped, so don't build the theme around those.
 
 You can have your theme added to [the extra themes page](https://omarchy.org/themes/) by sending a pull request to [the omarchy-site repo](https://github.com/omacom-io/omarchy-site).
diff --git a/test/shell.d/theme-install-guards-test.sh b/test/shell.d/theme-install-guards-test.sh
index 43cf1f3e..2b29bba8 100755
--- a/test/shell.d/theme-install-guards-test.sh
+++ b/test/shell.d/theme-install-guards-test.sh
@@ -120,6 +120,21 @@ grep -Fq "/themes/tokyo_night.2" "$git_calls" ||
 
 pass "a theme name may still hold an underscore, a dot, and a dash"
 
+# A plus is neither path-climb nor shell syntax, and a leading underscore is
+# neither the `..` climb nor the dash that reads as an option, so the allowlist
+# keeps both rather than stranding a repo that names itself with them.
+install_theme "https://github.com/example/omarchy-c++-theme.git" ||
+  fail "omarchy-theme-install accepts a name holding a plus"
+grep -Fq "/themes/c++" "$git_calls" ||
+  fail "omarchy-theme-install derives a name carrying a plus" "$(cat "$git_calls")"
+
+install_theme "https://github.com/example/_private.git" ||
+  fail "omarchy-theme-install accepts a name starting with an underscore"
+grep -Fq "/themes/_private" "$git_calls" ||
+  fail "omarchy-theme-install derives a name starting with an underscore" "$(cat "$git_calls")"
+
+pass "a plus and a leading underscore are still usable theme names"
+
 # basename reads a leading dash as an option once the scp-style prefix is gone.
 install_theme "host:-s/foo.git" || fail "omarchy-theme-install accepts a normal scp-style URL"
 grep -Fq -- "-- host:-s/foo.git" "$git_calls" || fail "omarchy-theme-install passes the URL after --" "$(cat "$git_calls")"