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Since the question of what constitutes a complex vs. a simple match is processor-specific, the safest way to ensure desired behaviour is through explicit priorities. Thus, for empty lists in our example, all thee three templates might match a given element, but the one designed to match the specific case wins because it is explicitly given a higher priority. If you look through the templates in the dri2xhtml.xsl file itself, you will notice many templates bearing a priority attribute to disambigute cases where their match sets intersect.
A problem may arise, however, when these templates are imported into another stylesheet, as is the case in all themes overriding templates from the dri2xhtml base libray. Because templates belonging to the importing stylesheet have higher priority than those that are being imported (which is what allows the original templates to be overridden in the first place), this can produce unintended consequences when overriding only the simple and low-priority templates. Using our example above, we cannot just override the template than matches all lists if we also wish to preserve the original behavior, because it will always override the more specific ones simply by the virtue of having higher import priority. For this reason, when overriding simple templates for a particular element, always make sure that the templates dealing with the more esoteric conditions for that element are imported as well, even if you do not intent intend to change them.
The last thing to point out here is the use of the "mode" attribute on templates. These are used to differentiate templates that apply to the exact same elements, usually to process the same element under different conditions. In order to match, a template's mode must correspond to the mode of the template call, so a call of the form <xsl:apply-template select="dri:list" mode="nested"/> will only match templates that have the same mode attribute, i.e. <xsl:template match="dri:list" mode="nested">. This pairing overrides all other templates, regardless of priority and import precedence, so it is generally advisable to preserve the modes in imported templates. If the modes are kept consisent, however, those templates are subject to the same disambiguation rules as templates with modes.
Positioning of theme
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declarations in the xmlui.xconf file
The order in which themes are listed in the xmlui.xconf file is important. When looking for a theme to apply to a DSpace page, the Theme Matcher will go through each theme declaration in order until it finds the first one that matches. If your theme is not getting applied to a specific community, collection or url, first make sure that the theme's declaration is not preceded by another, more general, one. It is generally a good idea to have the most specific matches come first (those for specific urls URLs or handles attached to items), followed by the more general ones attached to collections and communities, with the most general "default" theme coming last.
Trailing slashes on theme locations
A simple but common gocha gotcha is forgetting to follow all theme path paths with a trailing forward slash. This comes into play in two places: xmlui.xconf theme declarations and the use of the themepath parameter when dynamically switching out themes in a browser.