Update theme documentation

book/src/themes.md

@@ -13,10 +13,10 @@ The default theme.toml can be found [here](https://github.com/helix-editor/helix
 Each line in the theme file is specified as below:
 
 ```toml
-key = { fg = "#ffffff", bg = "#000000", modifiers = ["bold", "italic"] }
+key = { fg = "#ffffff", bg = "#000000", underline = "#ff0000", modifiers = ["bold", "italic", "undercurled"] }
 ```
 
-where `key` represents what you want to style, `fg` specifies the foreground color, `bg` the background color, and `modifiers` is a list of style modifiers. `bg` and `modifiers` can be omitted to defer to the defaults.
+where `key` represents what you want to style, `fg` specifies the foreground color, `bg` the background color, `underline` the underline color (only meaningful if an underline modifier is enabled), and `modifiers` is a list of style modifiers. `bg`, `underline` and `modifiers` can be omitted to defer to the defaults.
 
 To specify only the foreground color:
 
@@ -83,6 +83,10 @@ Less common modifiers might not be supported by your terminal emulator.
 | `dim`                |
 | `italic`             |
 | `underlined`         |
+| `undercurled`        |
+| `underdashed`        |
+| `underdotted`        |
+| `double-underlined`  |
 | `slow_blink`         |
 | `rapid_blink`        |
 | `reversed`           |