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+# Theming
+
+Since the widget toolkit is **GTK3** theming is done with **CSS**.
+
+- [CSS tutorial](https://www.w3schools.com/css/)
+- [GTK CSS Overview wiki](https://docs.gtk.org/gtk3/css-overview.html)
+- [GTK CSS Properties Overview wiki](https://docs.gtk.org/gtk3/css-properties.html)
+
+:::warning GTK is not the web
+While most features are implemented in GTK,
+you can't assume anything that works on the web will work with GTK.
+Refer to the [GTK docs](https://docs.gtk.org/gtk3/css-overview.html)
+to see what is available.
+:::
+
+So far every widget you made used your default GTK3 theme.
+To make them more custom, you can apply stylesheets to them.
+
+## From file at startup
+
+You can pass a path to a file or css as a string in `App.start`
+
+:::code-group
+
+```ts [app.ts]
+const inlineCss = `
+window {
+    background-color: transparent;
+}
+`
+
+App.start({
+    css: "./style.css",
+    css: `${SRC}/style.css'`,
+    css: inlineCss,
+})
+```
+
+:::
+
+:::info
+The global `SRC` will point to the directory `app.ts` is in.
+AGS will set the current working directory to `--config`, so `./style.css` also works.
+:::
+
+## Css Property on Widgets
+
+```ts
+Widget.Label({
+    css: "color: blue; padding: 1em;",
+    label: "hello",
+})
+```
+
+:::info
+The `css` property of a widget will not cascade to its children.
+:::
+
+## Apply Stylesheets at Runtime
+
+You can apply additional styles at runtime.
+
+```ts
+App.apply_css("/path/to/file.css")
+```
+
+```ts
+App.apply_css(`
+window {
+    background-color: transparent;
+}
+`)
+```
+
+```ts
+App.reset_css() // reset if need
+```
+
+:::warning
+`App.apply_css` will apply on top of other stylesheets applied before.
+You can reset stylesheets with `App.resetCss`
+:::
+
+## Inspector
+
+If you are not sure about the widget hierarchy or any CSS selector,
+you can use the [GTK inspector](https://wiki.gnome.org/Projects/GTK/Inspector)
+
+```sh
+# to bring up the inspector run
+ags --inspector
+```
+
+## Using SCSS
+
+Gtk's CSS only supports a subset of what the web offers.
+Most notably nested selectors are unsupported by Gtk, but this can be
+workaround by using preprocessors like [SCSS](https://sass-lang.com/).
+
+:::code-group
+
+```sh [Arch]
+sudo pacman -Syu dart-sass
+```
+
+```sh [Fedora]
+npm install -g sass # not packaged on Fedora
+```
+
+```sh [Alpine]
+sudo apk add dart-sass
+```
+
+```sh [Ubuntu]
+npm install -g sass # not packaged on Ubuntu
+```
+
+```sh [openSUSE]
+sudo zypper install dart-sass
+```
+
+:::
+
+Importing `scss` files will simply return transpiled css.
+
+:::code-group
+
+```ts [app.ts]
+import style from "./style.scss"
+
+App.start({
+    css: style,
+    main() {},
+})
+```
+
+:::
+
+:::tip
+If you for example want to set scss varibles from JS,
+You can inline import, compose, and transpile yourself.
+
+```ts [app.ts]
+import style1 from "inline:./style1.scss"
+import style2 from "inline:./style2.scss"
+
+const tmpscss = "/tmp/style.scss"
+const target = "/tmp/style.css"
+
+writeFile(tmpscss, `
+  $var1: red;
+  $var1: blue;
+  ${style1}
+  ${style1}
+`)
+
+exec(`sass ${tmpscss} ${target}`)
+
+App.start({
+    css: target,
+})
+
+```
+
+:::
+
+:::info
+If you want other preprocessors support builtin open an Issue.
+:::