fix: Prevent default theme from overriding custom themes

[Ralex91]

This PR fixes an issue where the default theme (elysiajsTheme) would override custom themes when config.customCss was not explicitly set to truly.

Before

The default theme was applied whenever config.customCss was falsy (null, undefined, or false):

<style>
  ${config.customCss ?? elysiajsTheme}
</style>

After

Now, the default theme is only applied if both config.customCss and config.theme are falsy:

<style>
  ${config.customCss || config.theme ? config.customCss : elysiajsTheme}
</style>

Why?

This prevents the default theme from overriding a manually selected theme when config.customCss is falsy but config.theme is set.