diff --git a/tests/rustdoc-gui/sidebar-source-code.goml b/tests/rustdoc-gui/sidebar-source-code.goml
index 69c589741cb..92b9045b734 100644
--- a/tests/rustdoc-gui/sidebar-source-code.goml
+++ b/tests/rustdoc-gui/sidebar-source-code.goml
@@ -73,7 +73,7 @@ assert: "//*[@class='dir-entry' and @open]/*[text()='sub_mod']"
 // Only "another_folder" should be "open" in "lib2".
 assert: "//*[@class='dir-entry' and not(@open)]/*[text()='another_mod']"
 // All other trees should be collapsed.
-assert-count: ("//*[@id='src-sidebar']/details[not(text()='lib2') and not(@open)]", 9)
+assert-count: ("//*[@id='src-sidebar']/details[not(text()='lib2') and not(@open)]", 10)
 
 // We now switch to mobile mode.
 set-window-size: (600, 600)
diff --git a/tests/rustdoc-gui/src/theme_css/Cargo.lock b/tests/rustdoc-gui/src/theme_css/Cargo.lock
new file mode 100644
index 00000000000..7ad6737a4d0
--- /dev/null
+++ b/tests/rustdoc-gui/src/theme_css/Cargo.lock
@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "theme_css"
+version = "0.1.0"
diff --git a/tests/rustdoc-gui/src/theme_css/Cargo.toml b/tests/rustdoc-gui/src/theme_css/Cargo.toml
new file mode 100644
index 00000000000..798e64f9309
--- /dev/null
+++ b/tests/rustdoc-gui/src/theme_css/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "theme_css"
+version = "0.1.0"
+edition = "2018"
+
+[lib]
+path = "lib.rs"
diff --git a/tests/rustdoc-gui/src/theme_css/custom-theme.css b/tests/rustdoc-gui/src/theme_css/custom-theme.css
new file mode 100644
index 00000000000..260ef87f6ea
--- /dev/null
+++ b/tests/rustdoc-gui/src/theme_css/custom-theme.css
@@ -0,0 +1,99 @@
+:root {
+	--main-background-color: red;
+	--main-color: black;
+	--settings-input-color: #2196f3;
+	--settings-input-border-color: #717171;
+	--settings-button-color: #000;
+	--settings-button-border-focus: #717171;
+	--sidebar-background-color: #f5f5f5;
+	--sidebar-background-color-hover: #e0e0e0;
+	--code-block-background-color: #f5f5f5;
+	--scrollbar-track-background-color: #dcdcdc;
+	--scrollbar-thumb-background-color: rgba(36, 37, 39, 0.6);
+	--scrollbar-color: rgba(36, 37, 39, 0.6) #d9d9d9;
+	--headings-border-bottom-color: #ddd;
+	--border-color: #e0e0e0;
+	--button-background-color: #fff;
+	--right-side-color: grey;
+	--code-attribute-color: #999;
+	--toggles-color: #999;
+	--toggle-filter: none;
+	--search-input-focused-border-color: #66afe9;
+	--copy-path-button-color: #999;
+	--copy-path-img-filter: invert(50%);
+	--copy-path-img-hover-filter: invert(35%);
+	--codeblock-error-hover-color: rgb(255, 0, 0);
+	--codeblock-error-color: rgba(255, 0, 0, .5);
+	--codeblock-ignore-hover-color: rgb(255, 142, 0);
+	--codeblock-ignore-color: rgba(255, 142, 0, .6);
+	--warning-border-color: #ff8e00;
+	--type-link-color: #ad378a;
+	--trait-link-color: #6e4fc9;
+	--assoc-item-link-color: #3873ad;
+	--function-link-color: #ad7c37;
+	--macro-link-color: #068000;
+	--keyword-link-color: #3873ad;
+	--mod-link-color: #3873ad;
+	--link-color: #3873ad;
+	--sidebar-link-color: #356da4;
+	--sidebar-current-link-background-color: #fff;
+	--search-result-link-focus-background-color: #ccc;
+	--search-result-border-color: #aaa3;
+	--search-color: #000;
+	--search-error-code-background-color: #d0cccc;
+	--search-results-alias-color: #000;
+	--search-results-grey-color: #999;
+	--search-tab-title-count-color: #888;
+	--search-tab-button-not-selected-border-top-color: #e6e6e6;
+	--search-tab-button-not-selected-background: #e6e6e6;
+	--search-tab-button-selected-border-top-color: #0089ff;
+	--search-tab-button-selected-background: #fff;
+	--stab-background-color: #fff5d6;
+	--stab-code-color: #000;
+	--code-highlight-kw-color: #8959a8;
+	--code-highlight-kw-2-color: #4271ae;
+	--code-highlight-lifetime-color: #b76514;
+	--code-highlight-prelude-color: #4271ae;
+	--code-highlight-prelude-val-color: #c82829;
+	--code-highlight-number-color: #718c00;
+	--code-highlight-string-color: #718c00;
+	--code-highlight-literal-color: #c82829;
+	--code-highlight-attribute-color: #c82829;
+	--code-highlight-self-color: #c82829;
+	--code-highlight-macro-color: #3e999f;
+	--code-highlight-question-mark-color: #ff9011;
+	--code-highlight-comment-color: #8e908c;
+	--code-highlight-doc-comment-color: #4d4d4c;
+	--src-line-numbers-span-color: #c67e2d;
+	--src-line-number-highlighted-background-color: #fdffd3;
+	--test-arrow-color: #f5f5f5;
+	--test-arrow-background-color: rgba(78, 139, 202, 0.2);
+	--test-arrow-hover-color: #f5f5f5;
+	--test-arrow-hover-background-color: rgb(78, 139, 202);
+	--target-background-color: #fdffd3;
+	--target-border-color: #ad7c37;
+	--kbd-color: #000;
+	--kbd-background: #fafbfc;
+	--kbd-box-shadow-color: #c6cbd1;
+	--rust-logo-filter: initial;
+	/* match border-color; uses https://codepen.io/sosuke/pen/Pjoqqp */
+	--crate-search-div-filter: invert(100%) sepia(0%) saturate(4223%) hue-rotate(289deg)
+		brightness(114%) contrast(76%);
+	--crate-search-div-hover-filter: invert(44%) sepia(18%) saturate(23%) hue-rotate(317deg)
+		brightness(96%) contrast(93%);
+	--crate-search-hover-border: #717171;
+	--src-sidebar-background-selected: #fff;
+	--src-sidebar-background-hover: #e0e0e0;
+	--table-alt-row-background-color: #f5f5f5;
+	--codeblock-link-background: #eee;
+	--scrape-example-toggle-line-background: #ccc;
+	--scrape-example-toggle-line-hover-background: #999;
+	--scrape-example-code-line-highlight: #fcffd6;
+	--scrape-example-code-line-highlight-focus: #f6fdb0;
+	--scrape-example-help-border-color: #555;
+	--scrape-example-help-color: #333;
+	--scrape-example-help-hover-border-color: #000;
+	--scrape-example-help-hover-color: #000;
+	--scrape-example-code-wrapper-background-start: rgba(255, 255, 255, 1);
+	--scrape-example-code-wrapper-background-end: rgba(255, 255, 255, 0);
+}
diff --git a/tests/rustdoc-gui/src/theme_css/lib.rs b/tests/rustdoc-gui/src/theme_css/lib.rs
new file mode 100644
index 00000000000..e9f3265fa6b
--- /dev/null
+++ b/tests/rustdoc-gui/src/theme_css/lib.rs
@@ -0,0 +1,2 @@
+// compile-flags: --theme custom-theme.css
+//! <div class="custom-text">custom text</div>
diff --git a/tests/rustdoc-gui/theme-change.goml b/tests/rustdoc-gui/theme-change.goml
index e4b031b735e..fdaf9d6289a 100644
--- a/tests/rustdoc-gui/theme-change.goml
+++ b/tests/rustdoc-gui/theme-change.goml
@@ -65,3 +65,36 @@ assert-local-storage: { "rustdoc-theme": "light" }
 reload:
 wait-for: "#settings"
 assert: "#preferred-light-theme.setting-line.hidden"
+
+// Ensures that the custom theme feature is working as expected.
+go-to: "file://" + |DOC_PATH| + "/theme_css/index.html"
+set-local-storage: {"rustdoc-use-system-theme": "false", "rustdoc-theme": "dark"}
+reload:
+
+store-value: (background_light, "white")
+store-value: (background_dark, "#353535")
+store-value: (background_ayu, "#0f1419")
+store-value: (background_custom_theme, "red")
+
+click: "#settings-menu"
+wait-for: "#theme-ayu"
+click: "#theme-ayu"
+// should be the ayu theme so let's check the color.
+wait-for-css: ("body", { "background-color": |background_ayu| })
+assert-local-storage: { "rustdoc-theme": "ayu" }
+assert-text: (".custom-text", "custom text")
+click: "#theme-light"
+// should be the light theme so let's check the color.
+wait-for-css: ("body", { "background-color": |background_light| })
+assert-local-storage: { "rustdoc-theme": "light" }
+assert-text: (".custom-text", "custom text")
+click: "#theme-dark"
+// Should be the dark theme so let's check the color.
+wait-for-css: ("body", { "background-color": |background_dark| })
+assert-local-storage: { "rustdoc-theme": "dark" }
+assert-text: (".custom-text", "custom text")
+click: "#theme-custom-theme"
+// Should be the custom theme so let's check the color.
+wait-for-css: ("body", { "background-color": |background_custom_theme| })
+assert-local-storage: { "rustdoc-theme": "custom-theme" }
+assert-text: (".custom-text", "custom text")