mimi1911

It’s been a long time since I have written any CSS but I’m fairly sure that the upper/lower case issue is purely a personal – or organisational – choice. Mimi’s teacher might personally hate using upper case hex values but if Mimi goes to work for a company the practices of that company may specify that upper case is actually preferred (for whatever reason).
A good rule-of-thumb here would seem to be: Be consistent with the rest of the code base.
In other words, if you are writing it just for yourself then do whatever feels best for you, but if you have to work with other people, do what they do.
The W3C specification https://www.w3.org/TR/CSS2/syndata.html#characters states that “All CSS syntax is case-insensitive within the ASCII range (i.e., [a-z] and [A-Z] are equivalent), except for parts that are not under the control of CSS.” and the examples for colour hex values (section 4.3.6) are in both lower and upper case, so the W3C don’t seem to be too bothered about it.

Depending on where you write your code, something like this https://stylelint.io/ might be useful, e.g. https://stylelint.io/user-guide/rules/color-hex-case

No there is no way to set the letters to lower case from within the Affinity apps, unless you pasted to the workspace and changed the case of the letters and then copied to clipboard again.
 

For CSS/HTML coding one usually also use or have a capable text editor beside, just use that then and it's regex find and replace with convert case feature for those files.