Added Gitea Theming Guide

GiteaThemes.md

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+# Gitea Theming
+## Theme Directory
+- Find where your $CUSTOMPAATH$ directory is, mine is `/data/gitea/public/css`.
+## Gitea Version 1.18+
+### [Sainnhe themes](https://git.sainnhe.dev/sainnhe/gitea-themes)
+- Navigate to the theme directory and using `wget` you can download any of the raw files of the css theme
+  - Example for Nord: `wget https://git.sainnhe.dev/sainnhe/gitea-themes/raw/branch/master/dist/theme-nord.css`
+- Once the file is copied, use your favorite editor to open the file.(Taken from changes of the official `arc-green` theme)
+  - Search for these values of the theme.
+```css
+--color-navbar: #222222;
+--color-navbar-transparent: #22222200;
+--color-nav-bg: unset;
+--color-reaction-active-bg: var(--color-primary-alpha-40)
+```
+    - Replace the lines with the new values.
+```CSS
+--color-header-wrapper: #222222;
+--color-header-wrapper-transparent: #22222200;
+--color-nav-bg: #222222;
+--color-nav-hover-bg: #303030;
+--color-reaction-active-bg: var(--color-primary-alpha-40);
+--color-label-text: #dbe0ea
+```
+- Once the file has been changed, navigate to the config file `app.ini`
+  - Mine is located in `/data/gitea/confs`
+  - Find the `[ui]` section and add in the name of the new theme to `THEMES`.
+```ini
+...
+[ui]
+THEMES = auto,gitea,arc-green,<your theme>
+DEFAULT_THEME = <your theme>
+...
+```
+- Save the file and restart the Gitea instance
+- Navigate to the appearance section under your own account and change the theme.
+![](images/2023-08-04_03.11.04AM.png)