Allow custom CSS theme/customization with 1 line

[pachadotdev]

Dear @wch

I hope this message finds you and your beloved ones in good health.

Since 2018 I've been using a customized shinydashboard, and today I've brought the option to use a custom CSS in 1 line here, to the mainstream version.

At first, I used some codes by @gadenbuie, this is why I listed him, then I felt motivated and polished it until it was ok to make shiny.tradestatistics.io win an honourable award in the Shiny Contest 2020.

These changes are harmless in a way that:

1. Don't alter the order of current arguments
2. Add optional arguments that integrate with the existing code
3. Doesn't break dashboards running without custom CSS options

As an example, the aforementioned dashboard uses these simple changes
https://github.com/tradestatistics/visualization-with-shiny/blob/master/ui.R#L6-L7
https://github.com/tradestatistics/visualization-with-shiny/blob/master/global.R#L103-L107

I added a MWE here https://github.com/pachadotdev/shinydashboard/tree/main/custom-theme-vs-regular-theme. This example presents all the CSS customizations I've imagined, and works with smaller CSS customization (i.e. copy paste skin-blue from https://github.com/pachadotdev/shinydashboard/blob/main/inst/AdminLTE/_all-skins.css#L5-L139 and put it in a new CSS into www/).

Before creating this PR I run a regular devtools::check() and I see no problems

> check()
...
── R CMD check results ──────────────────────────────────────────────────────────────────────────────────────────────────── shinydashboard 0.7.2 ────
Duration: 13.3s

0 errors ✓ | 0 warnings ✓ | 0 notes ✓

[pachadotdev]

Hi @wch , I hope this PR is still relevant

[CLAassistant]

All committers have signed the CLA.

[YongbingDing]

您好，丁永兵已经收到您的邮件，谢谢。

[pachadotdev]

您好，丁永兵已经收到您的邮件，谢谢。

对不起，我已经很多年没有练习过普通拳了。我们可以试着在这里写一些字进行交流。