Theme Documentation (#3415)

* adding self CREDITS.md

* add "theme" to link for easier navigation

* add documentation about themes and the files that go in them

* add admin documentation for themes

* fix markdown styling

* fix CSSJanus usage

1 files changed, 30 insertions, 5 deletions

diff --git a/docs/en/developers/04_Frontend/02_Design.md b/docs/en/developers/04_Frontend/02_Design.md
index c2e622a08..625fede28 100644
--- a/docs/en/developers/04_Frontend/02_Design.md
+++ b/docs/en/developers/04_Frontend/02_Design.md
@@ -1,11 +1,36 @@
-# Template file
+# Writing a new theme
 
-**TODO**
+**Note: Currently personal themes are not officially supported and may be overwritten when updating. Be sure to keep backups!**
 
-# Writing a new theme
+**As of writing (02-02-2021), support for themes as extensions is under development.**
+
+The easiest way to create a theme is by copying and modifying the base theme (or another of the pre-installed themes). The base theme can be found in [/p/themes/base-theme](https://github.com/FreshRSS/FreshRSS/tree/master/p/themes/base-theme). Themes require:
+- a **metadata.json** file to describe the theme.
+- a **loader.gif** file to use as a loading icon (assuming .loading's background has not been overridden)
+- an **icons** folder containing .svg, .ico, and .png files to override existing icons
+- a **thumbs** folder containing a file, **original.png** that will be used as the preview for the theme
+
+"_template.css" is normally added to the metadata file as a fallback for missing aspects. The file is taken from the base theme. If submitting a pull request for a theme, please know that [pull request themes must include this file.](https://github.com/FreshRSS/FreshRSS/pull/2938#issuecomment-624085450)
 
-**TODO**
+## RTL Support
+
+RTL (right-to-left) support for languages such as Hebrew and Arabic is handled through CSSJanus. To generate an RTL CSS file from your standard file, use `make rtl`. Be sure to commit the resulting file (filename.rtl.css).
 
 # Overriding icons
 
-**TODO**
+To replace the default icons, add an "icons" folder to your theme's folder. Use files with the same name as the default icon to override them. 
+
+# Template file
+
+metadata.json
+```
+{
+  "name": "Theme name",
+  "author": "Theme author",
+  "description": "Theme description",
+  "version": 0.1,
+  "files": ["_template.css", "file1.css", "file2.css"]
+}
+```
+
+An example of a css theme file can be found at [/p/themes/base-theme/base.css](https://github.com/FreshRSS/FreshRSS/blob/master/p/themes/base-theme/base.css)