docs: themes (#2425)

* docs: themes

* update

* update 2

* bigger images

docs/THEMES.md

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+# App color themes
+
+Mobile apps allow to configure, export and import color themes. Currently this is only supported in Android app.
+
+You can contribute your themes to the repository by creating a pull request.
+
+## How to contribute a theme
+
+1. Once you have configured your theme in the app, export it to a file and give it a descriptive name – e.g., `example.theme`
+
+2. Export your app database, and import a [sample chat database](./themes/simplex-chat.sample.zip).
+
+3. Make three screenshots - the list of conversations with opened profile picker, conversation and privacy settings.
+
+4. Create PR that includes these files and amends this THEMES.md file, following the example below.
+
+5. Restore your database from the backup.
+
+## Color themes
+
+### SimpleX - included dark blue theme
+
+Download [SimpleX theme](./themes/example.theme)
+
+<img src="./themes/example-chats.png" width="240"> &nbsp;&nbsp; <img src="./themes/example-conversation.png" width="240"> &nbsp;&nbsp; <img src="./themes/example-settings.png" width="240"> &nbsp;&nbsp;

docs/themes/example.theme

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+base: "SIMPLEX"
+colors:
+  accent: "#ff70f0f9"
+  accentVariant: "#ff1298a5"
+  secondary: "#ff8b8786"
+  secondaryVariant: "#ff2c464d"
+  background: "#ff111528"
+  menus: "#ff121c37"
+  title: "#ff267be5"
+  sentMessage: "#1e45b8ff"
+  receivedMessage: "#20b1b0b5"