fix($core): use stylus conditional assignment (close #2127)

[meteorlxy]

Summary

close #2127

What kind of change does this PR introduce? (check at least one)

• Bugfix ?
• Feature
• Code style update
• Refactor ?
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

Other information:

In fact, it's not a bug, but something can be enhanced.

[billyyyyy3320]

I suggest to change the commit message to something like fix($core): make default preset variables overridable (close #2127) to explain 'what' instead of 'how'.

[meteorlxy]

Yeah I agree with your point @newsbielt703 .

This PR is a little different, because the variables are already overridable. I just change the way to make the override looks better. (which fix an edge case in #2127 , but I think it's not a commonly used way)

So maybe it should be categorized as "refactor" rather than "fix"?

[billyyyyy3320]

@meteorlxy Totally agree 😉