Microsoft IIS caches all the style sheets that are placed inside the
folder, remembering the ones that are used by the web site pages (by all pages,
not only the current one); the effect is easily visible looking at the HTML code
sent to the client, noting that are included CSS links that were not specified
for the page.
This behavior requires a strong attention on styles inheritance, but also can
make Internet Explorer versions prior to the 9 to overflow the maximum number of
style tags allowed: in these versions the maximum number of style tags is 32 and
all the following ones are ignored.
My personal advise is to use the
App_Themes folder only and
exclusively for themes and to use another one, for example
for style sheets.
As anticipated, if the
App_Themes folder is needed, you have to pay
attention to styles inheritance, always resetting properties, for example:
The result will be:
top inheritance from the cache, the included style
sheet has to be: