You might try experimenting with some of the available themes via the Addon menu

re :Do the Thunderbird developers go out of their way to make the look and feel of Thunderbird ugly.

Whether the design colouration is perceived as Ugly or not is a matter of what each individual feels. Some like masses of colour all over the place and others do not. Adding colour is not as simple as it seems because we all like something different. But the biggest problem is that many people suffer from colour blindness or poor eyesight and that also takes various forms. So the developers have to come up with a design that is user inclusive.

In 'Add-ons and Themes', Select 'Themes' The defaults are a 'Light' version, a 'Dark version and a 'System theme' version.

The 'System theme' will use some elements from whatever you have set up on computer. for example - In 'Windows OS' in computer 'Settings' > 'Personalisation' > 'Colours' under 'Choose your colour' and select 'Custom' This allows you to choose a 'Light' or 'Dark' default app mode.

you can also eg: 'Choose your accent colour' - I have selected 'Seafoam teal' I have also selected the checkbox 'Titlebars and window borders'. So in Thunderbird using 'system theme', the title bar, windows borders, text box borders, icon highlights, blue buttons, and a few other options now use the 'Seafoam teal' colour. Image below as example.

If you need to use 'High contrast settings' or set up your own colourisation for the computer desktop then this can also effect what you see in Thunderbird. I've just quickly done a 'sandy' high contrast theme - see image below - it's not perfect as it's a fast knock up but it shows what sort of colourisation can be done just using computer settings.

This means some alterations can be fully controlled by your customising choices, so allowing you to add a little colour to some elements.

If you feel you could attempt to get more personalisation and control, but do not want to alter the general computer set up eg: via the High contrast then there is also a means via a 'userChrome.css' file.

A Basic example of this is found at the following link: https://support.mozilla.org/en-US/questions/1425743

I have altered my version of Thunderbird via the userChrome.css file and it looks like the image below.

However, all that might seem too much bother to work it all out.

The method advised by david is a much easier option.

In 'Addons and Themes' Select 'Themes' and type 'themes' in the top search A selection will appear in the list, but it might seem a bit daunting as there could be a couple of hundred options, so try typing eg: the word 'blue' in the addons search so it returns a load of blue theme options or type something like 'flowers' and you get various floral themes.

Please note - all addons are designed by other users called 'authors' and not by 'Thunderbird'. If there is an issue with any theme, it is not for Thunderbird to fix and you would need to inform the author about the problem.