refactor: add @radio-button-checked-bg

[willc001]

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Merging #15541 into master will not change coverage.
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[zombieJ]

Maybe change .@{radio-group-prefix-cls}-solid background color instead?
Since we have provided the solid radio: https://ant.design/components/radio-cn/#components-radio-demo-radiobutton-solid

[afc163]

@radio-button-bg is what you wanted.

[afc163]

I believe @radio-button-bg is what you wanted, no need to change it's name.

[willc001]

I'm sorry for not explaining clearly. @radio-button-bg does change the background color of selected radio button. But it will also change the background color of unselected radio buttons and normal radios, which is not expected. @afc163

Expected:

Actually:

[afc163]

@willc001 Thx for explaination, PR is accepted.