[feature/themes] Add auto-switching themes for blurple/brutalist/sola…

…rized (#3588)

web/assets/themes/blurple-auto.css

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+/*
+  theme-title: Blurple (auto)
+  theme-description: Official blurple theme that adapts to system preferences
+*/
+
+/* Default to dark theme */
+@import url("blurple-dark.css");
+
+@import url("blurple-light.css") screen and (prefers-color-scheme: light);
+@import url("blurple-dark.css") screen and (prefers-color-scheme: dark);

web/assets/themes/brutalist-auto.css

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+/*
+  theme-title: Brutalist (auto)
+  theme-description: Official (Pseudo-)monochrome brutality theme that adapts to system preferences
+*/
+
+/* Default to brutalist theme */
+@import url("brutalist.css");
+
+@import url("brutalist.css") screen and (prefers-color-scheme: light);
+@import url("brutalist-dark.css") screen and (prefers-color-scheme: dark);

web/assets/themes/solarized-auto.css

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+/*
+  theme-title: Solarized (auto)
+  theme-description: Solarized theme that adapts to system preferences
+*/
+
+/* Default to dark theme */
+@import url("solarized-dark.css");
+
+@import url("solarized-light.css") screen and (prefers-color-scheme: light);
+@import url("solarized-dark.css") screen and (prefers-color-scheme: dark);