Location of arrow (Design/Themes)
If I create my own spirit image to replace bg_sprite_1.png, do I have to duplicate the size of bg_sprite_1.png? Is the size defined in the CSS reference?
If you don't want to maintain a separate theme, you have to respect the dimensions of the default-theme-images. Your changes wold be overwritten in case of an update. Or you have to keep in mind to adapt your changes (different images and dimension rules in the CSS file) to the default theme in case of a forum update.
Otherwise it would be a good idea to fork the default theme (or another one) into an own theme, where you can use your own sprite sizes. Also in this case you have to adjust the CSS rules for all sprites that are located in a sprite-set-image and you might to have to adapt changes in the default theme into your own one when an update makes this necessary.
For the thick arrow for a new entry this is
background:url(images/bg_sprite_1.png) no-repeat 0 4px;, where
0 4px names the left top corner of the displayed section. You can see a few exemplary definitions starting with line #24 of
style.css of the default theme.
The actually used CSS file
style.min.css contains the same rules in a more condensed notation but it is not necessary to use a minified version. The minified CSS file will use a lower amount of the bandwidth when loaded from the server, but counter-arguments are the additional effort to create the file after every change (one has to maintain two files) and the poor readability.
I.e. my own theme makes no use of a minified version of the CSS file. The size is only 1 1/3 the size of the minified version of the default-CSS with many more rules in the current state and most of the servers (as mine) compress flat text files good enough in every case. So I (for myself) see no need for a minification of the CSS file.
Trenne niemals Müll, denn er hat nur eine Silbe!