feat(theme): add github (@KevinRnbrg) (monkeytypegame#6193)

### Description

Added new theme "github" with colors used from GitHub.

### Checks

- [ ] Adding quotes?
- [ ] Make sure to include translations for the quotes in the
description (or another comment) so we can verify their content.
- [x] Adding a language or a theme?
- [ ] If is a language, did you edit `_list.json`, `_groups.json` and
add `languages.json`?
  - [x] If is a theme, did you add the theme.css?
- Also please add a screenshot of the theme, it would be extra awesome
if you do so!
- [ ] Check if any open issues are related to this PR; if so, be sure to
tag them below.
- [x] Make sure the PR title follows the Conventional Commits standard.
(https://www.conventionalcommits.org for more info)
- [x] Make sure to include your GitHub username prefixed with @ inside
parentheses at the end of the PR title.

Theme screenshots:

![typing](https://github.com/user-attachments/assets/c5343d2c-4070-4839-ac06-38468cc9a294)

![result](https://github.com/user-attachments/assets/164e0c61-1d98-4b14-933a-c4c2dd62e913)

![settings](https://github.com/user-attachments/assets/8d051708-8df5-42f8-83e2-aecf8ee854ee)

![profile_1](https://github.com/user-attachments/assets/40c481e6-43a9-4cfd-afd4-5097e680dabb)

![profile_2](https://github.com/user-attachments/assets/0446777f-e04f-4b1b-9f0c-2f1fdef90c2e)

Author: KevinRnbrg

frontend/static/themes/_list.json

@@ -1251,5 +1251,12 @@
     "mainColor": "#bef0ff",
     "subColor": "#fe9841",
     "textColor": "#dbdeeb"
+  },
+  {
+    "name": "github",
+    "bgColor": "#212830",
+    "mainColor": "#41ce5c",
+    "subColor": "#788386",
+    "textColor": "#ccdae6"
   }
 ]

frontend/static/themes/github.css

@@ -0,0 +1,12 @@
+:root {
+  --bg-color: #212830;
+  --main-color: #41ce5c;
+  --caret-color: #41ce5c;
+  --sub-color: #788386;
+  --sub-alt-color: #141b23;
+  --text-color: #ccdae6;
+  --error-color: #c23e3a;
+  --error-extra-color: #c23e3a;
+  --colorful-error-color: #c23e3a;
+  --colorful-error-extra-color: #c23e3a;
+}