add modules gruvbox, rainbow

modules/readmes/gruvbox.txt

@@ -0,0 +1,5 @@
+in the css
+
+@import url("https://styles.alv.cx/modules/gruvbox.css");
+
+and use the variables created in the file

modules/readmes/rainbow.txt

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+in the css
+
+@import url("https://styles.alv.cx/modules/rainbow.css");
+
+and add the class `rainbow` to any element you want.
+
+it works by changing the `color` property of the element.
+
+the duration is determined by the variable `--rainbow-duration`