Customized SCSS/CSS

The user should be able to change the colors of the theme without modifying the theme itself. Thus it is very important to implement a way to use a customized stylesheet. It should address two possible situations:

## User that prefers to use CSS directly

If the user will prefer to use CSS dircetly, it should have instructions about how to modify each thing in a custom stylesheet. The user should know the the pros and cons of this way:

- **Pros**. You can edit and use directly your CSS.
- **Cons**. It may be more complex to customize all Buruma.

## User that prefers to use SCSS

If the user will prefer to use SCSS, the user would need to know a good way to convert SCSS into a CSS file. The user should know the pros and cons of this way:

- **Pros**. User can benefit of all the SCSS extension.
- **Cons**. User will need to convert SCSS to CSS to use it.
- **Pros**. Related with the cons, the process to build a CSS file from the SCSS might be easier investing a bit of time with the installation and configuring of some packages and scripts (take a look at [package.json](https://github.com/ivanhercaz/buruma/blob/ad1532378367227ac41cda79b18ef7ca377805ec/package.json#L5-L8)).
- **Cons**. Whatever the user uses, the user will need an extra configuration to work with SCSS.

## Development

- [ ] Add way to integrate `CUSTOM_CSS` in `base.html` template: if `CUSTOM_CSS` exists, then add this ``.

## Documentation

- [ ] Document `CUSTOM_CSS`.
- [ ] Pros and cons.
- [ ] How-to add modifications to the theme.
- [ ] Document `CUSTOM_SCSS`.
- [ ] Pros and cons.
- [ ] How-to add modifications to the theme.
- [ ] How-to build the CSS.
- [ ] An easy solution: show how Buruma does it internally and how the user can apply it to its own site.

Start with base.html for the proposed CUSTOM_CSS link and inspect package.json for the existing SCSS build setup. Document CUSTOM_CSS and CUSTOM_SCSS, including their pros and cons, theme modification steps, and how to build CSS from SCSS. Done means the custom stylesheet integration and both documentation checklists are complete.

Written by the indexing model from the issue text.