Lowercase matching the theme name

[deg4uss3r]

Using .to_lowercase() on the theme matching to avoid needing exact capitalization in the book.toml. Fixes #1078

[ehuss]

I'm a little concerned that this would make it impossible to use a theme that actually has an uppercase character in it. However, that seems unlikely to me.

[deg4uss3r]

@ehuss Doesn't the PR #804 get the themes and then lower case them which is why this is necessary?

https://github.com/rust-lang/mdBook/blob/master/src/renderer/html_handlebars/helpers/theme.rs#L23

[ehuss]

@deg4uss3r I think that lowercase only applies to showing the (default) indicator. The CSS class name is used directly as-is, and I believe those are case sensitive.

[deg4uss3r]

@ehuss Ah, I think you're right, here seems to be the real cause to my confusion making the list of themes capitalized which is what I put into the config, rather than the true matching lowercase value.

Do you think this solution interferes with the actual style sheet selection? If so, I'm happy to rework it to get the config option to be case-insensitive if I can be pointed in the right direction.

[ehuss]

Do you think this solution interferes with the actual style sheet selection? If so, I'm happy to rework it to get the config option to be case-insensitive if I can be pointed in the right direction.

No, I think it's fine. Adding a theme is a nontrivial task, and I don't suspect people use capitalized css names much anyways.