Fixes #2741: Update form inputs derived variables

[marcbelletre]

Use $grey-darker and $grey as default instead of $text-strong and $text-light which may contain the "inherit" value (fixes issue #2741)

This is an improvement / bugfix.

I ran into a compiling error with Bulma 0.8.0 when trying to use inherit value for the $text-strong variable. I found out this is due to breaking changes in commit 0f988ea in which a lot of default colors have been modified. This particular line was throwing the error when using inherit value for $text-strong:

$input-placeholder-color: rgba($text-strong, 0.3) !default

I think we may often use this particular value for the $text-strong and $text-light variables because there are a lot of cases where a <strong> tag in a text should just inherit its parent color.
Also I think it doesn't really make sense to use the strong text color as default in any other derived variable.

Proposed solution

In this PR I just put back the old default values, replacing $text-strong by $grey-darker and $text-light by $grey. By the way this is what the docs actually says.

Tradeoffs

There are still other derived variables which use the $text-strong and $text-light ones as default. All of these changes have been made in commit 0f988ea.
Maybe it could be a better idea to roll back the whole commit but I'm not really sure about that.

Testing Done

I did test this PR in my project. Using $text-strong: inherit does not throw a compiling error anymore.

Changelog updated?

No.

[afwn90cj93201nixr2e1re]

@jgthms

[jgthms]

The idea with using $text-strong is that you only need to update once if you want to make a dark mode for example.

The fix would be to instead handle the rgba() case where inherit is used.

[tomekr]

@jgthms with the use of inherit (https://github.com/jgthms/bulma/blob/0.8.1/sass/form/shared.sass#L1) it looks like in a hero, the input will inherit the text color from a heading/subheading, causing the text to be white.