Improve styles

[yuku]

• Introduce $qiita-slide-mode- prefix to Sass variables. Global variables always face to collision problem. Seriously.
• All package variables should be declared as !default.
• Remove unused rules.
• Enable to customize z-index of .fullscreen. Proper z-index value strongly depends on where this package runs on.

[potato4d]

@yuku-t

The user is supposed to read _core.scss only (it is described in the README)
The scss variable is a setting to enable external injection, but is it necessary to change the variable name on it?

[yuku]

The user is supposed to read _core.scss only (it is described in the README)

README describes as follows:

Ok, let's suppose you are right (in other words README is wrong), then where the following $break-point-xs comes from? Who defines it? To import the _core.scss, users must define $break-point-xs variable isn't it?

qiita-slide-mode/src/styles/_core.scss

Line 86 in a34825d

@media (max-width: $break-point-xs) {

[potato4d]

@yuku-t

Sorry. It was supposed to be the default value.

How about setting _vars as the default value and specifying in the README that if you want to expand it, ask that you overwrite it?

[potato4d]

Since it is considered that it is good to change the name of course if it is set as the default value, I think that it is good in the direction of the variable name change.

[yuku]

It was supposed to be the default value.

This is the reason why I set !default on them.

How about setting _vars as the default value and specifying in the README that if you want to expand it, ask that you overwrite it?

I think there is no difference between your opinion and this PR.

Suppose I want to override $qiita-slide-mode-fullscreen-z-index only, I can write:

$qiita-slide-mode-fullscree-z-index: 10;

@import "~@increments/qiita-slide-mode/src/styles/vars";
@import "~@increments/qiita-slide-mode/src/styles/core";