The thread-view is using Webkit to render the messages, the style and layout can be customized by editing the HTML and SCSS files.

You can copy the standard HTML and SCSS file from the ui/ directory to ~/.config/astroid/ui/ and then customize them to your own needs. Astroid will check if the version is correct (do not change it yourself), in case a new version is released upstream.

SCSS works much like CSS, but it is preprocessed at startup using libsass.

You can change font-size, font-family and some other variables for the whole style by just setting the SCSS variables at the top of thread-view.scss and part.scss.

If you only want to change the standard variables, but still use the default theme and follow any updates made upstream, create a ~/.config/astroid/ui/thread-view.scss and/or ~/.config/astroid/ui/part.scss file, set the variables, and then include the standard file:

Remember to keep the version line at the top intact!

/* ui-version: 5 (do not change when modifying theme for yourself) */
/* Variables */
/* Background colors associated with received emails: */
$recv-normal: #ffffff;
$recv-quoted: #e8e8e8;
$recv-collapsed: #f5f5f5;
/* Background colors associated with sent emails: */
$sent-normal: #ffffff;
$sent-quoted: #e8e8e8;
$sent-collapsed: #f5f5f5;
/* Fonts */
$font-base-size: 13px;
$font-mono: "Input Mono", "Inconsolata", monospace;
$font-sans: "Input Sans", "Roboto", sans-serif;
$font-family-default: $font-mono;
/* Colours */
$email-solid: #d8d8d8;
$focused-solid: #4a90d9;

@import '/path/to/upstream/astroid/ui/thread-view.scss';

The same is possible for part.scss which defines the look of the messages' body parts.

As upstream sources you can either use the git clone directory if you compiled manually, or /usr/share/astroid/ui if you installed it with root permissions, e.g. via your package manager.

Check the configuration options in ~/.config/astroid/config

You can customize the date format in Thread Index with three settings in your config file.

general.time.clock_format takes the options 24h (default), 12h, local and year. Using year you can avoid dates like "Yesterday" or "2h ago". Using local it uses your computer's default locale date format.

With general.time.same_year and general.time.diff_year you can configure the look of the dates for the current year and for all other years. It takes variables like the ones from GNU/Linux's date command. For example %d.%m. %H:%M will look like 29.08. 18:50 on August 29 at 18:50 in the current year. Using %x may be well suited for different years, since it looks like 29.08.2015.

Please be aware that the look of dates depend on your computer's locale, especially when it comes to the 24/12h clock format or the placement of days, months and years for sequences like %x.