Just list some issues you may like to know:

How to install theme for SunBlogNuke?
Connected your website root via FTP. Navigate to the $yourwebsite/DesktopModules/SunBlog directory. Locate the themes directory. This is the folder that contains all the themes for SunBlogNuke. Then just need to zip the target theme package and upload it there. That is all.
How to change themes? I've made some changes to the default theme for css styling and don't want it overwritten on the next upgrade... figured I would copy the default out and give it a new name but I can't find anywhere in the settings where I assign an existing blog to a new theme?
You can switch it in the edit blog panel. Now you can configure it from the 'theme' tab of customize panel in the latest version 4.2.5 and future version.
How to remove meta 'posted in [xx] by xx' in the basic/detail view?
Just find out the post meta, similar the code below:

<div class="post-meta"><%=LocalizeByKey("PostedIn")%>  <%=CategoryLinks(", ")%>  <i>By <b><%=Entry.Author%></b></i></div>

,  in the view file of current theme and delete it. 

How to modify the text '<%=Entry.PostedTime%>'  in the basic/detail view? I don’t want it showing the time, only the date.
Just replace it with the code below:

<%=FormatPostedTimeWithDayNumberSuffix()%> (that format actions in our team blog) or <%=FormatPostedTimeWithDayNumberSuffix("yyyy-MM-dd")%> (yyyy-MM-dd is the format supported in the .net framework).

How to show rating in the basic/detail view of our customized theme?
First check out the top whether there is declaration like the below:

<%@ Register TagPrefix="uc" TagName="Rating" Src="~/DesktopModules/SunBlog/Shared/Rating.ascx" %>

If not please append it. Then place the following code in anywhere as you want (note that you can’t modify its id otherwise it will throw error):

<uc:Rating ID="widgetRating" runat="server" EnableViewState="false"/>

BTW, you can refer to the _default theme and copy the rating section into your current theme.

How do I change the default text for
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Please refer to the tutorial for more details - Modify Some Default Words. Maybe you need to find out the resource keys like 'CategoryView_Title', 'TagsView_Title'  & 'TagsView_Description'  in the DNN core localization panel.