docs: Fix missing colons and comma in THEMES.md (@NeoNyaa) (#6707)

### Description

Fix missing colons and a single missing comma in the typescript
codeblock for the theme preview
#### Before

![Before](https://github.com/user-attachments/assets/0658f777-755f-46a0-9dbd-e8d7a11a8759)
#### After

![After](https://github.com/user-attachments/assets/48e4ec7d-af29-4b40-b3a4-2cb4dc752632)


### Checks

- [ ] Adding quotes?
- [ ] Make sure to include translations for the quotes in the
description (or another comment) so we can verify their content.
- [ ] Adding a language?
- Make sure to follow the [languages
documentation](https://github.com/monkeytypegame/monkeytype/blob/master/docs/LANGUAGES.md)
  - [ ] Add language to `packages/contracts/src/schemas/languages.ts`
- [ ] Add language to exactly one group in
`frontend/src/ts/constants/languages.ts`
  - [ ] Add language json file to `frontend/static/languages` 
- [ ] Adding a theme?
- Make sure to follow the [themes
documentation](https://github.com/monkeytypegame/monkeytype/blob/master/docs/THEMES.md)
  - [ ] Add theme to `packages/contracts/src/schemas/themes.ts`
  - [ ] Add theme to `frontend/src/ts/constants/themes.ts`
  - [ ] Add theme css file to `frontend/static/themes`
- Also please add a screenshot of the theme, it would be extra awesome
if you do so!
- [ ] Adding a layout?
- [ ] Make sure to follow the [layouts
documentation](https://github.com/monkeytypegame/monkeytype/blob/master/docs/LAYOUTS.md)
  - [ ] Add layout to `packages/contracts/src/schemas/layouts.ts`
  - [ ] Add layout json file to `frontend/static/layouts` 
- [ ] 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.

docs/THEMES.md

@@ -53,10 +53,10 @@ Then, go to `./frontend/src/ts/constants/themes.ts` and add the following code t
 export const themes: Record<ThemeName, Omit<Theme, "name">> = {
   ...
   your_theme_name: {
-    bgColor "#ffffff",
-    mainColor "#ffffff",
-    subColor "#ffffff",
-    textColor "#ffffff"
+    bgColor: "#ffffff",
+    mainColor: "#ffffff",
+    subColor: "#ffffff",
+    textColor: "#ffffff",
   },
 }
 ```