fix(css-base): use content_ instead of content (#674)

[Arvnox]

What kind of change does this PR introduce?
Bugfix

Did you add tests for your changes?
No, there isn't any new functionality.

If relevant, did you update the README?
Not relevant.

Summary
When using the library in WebSphere Portal within a ScriptPortlet, it throws an exception because the name of a local variable (inside a function) is a reserved word in the WCM of WebSphere Portal. This is the reported issue:
#674

Does this PR introduce a breaking change?
No, since it's a local variable.

Other information

[jsf-clabot]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

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[alexander-akait]

Why? Can you provide minimum reproducible steps?

[michael-ciniawsky]

If this is only specific to your particular setup, I have to decline this PR, but curious if this is/can be made a general case/problem

[michael-ciniawsky]

You also in any case need to sign the CLA please (close and reopen the PR to trigger the CLA Bot again)

[alexander-akait]

@michael-ciniawsky let's close this PR

[michael-ciniawsky]

@evilebottnawi The stage is yours 😛 (=> feel free to close it)

[alexander-akait]

Declined. Please search other solution to solve this problem. Sorry we should not change code for specific use case. In other env _content variable also may be a reserved word.