\documentclass{beamer}
\usepackage{tcolorbox}
\begin{document}
\begin{frame}[t]
\frametitle{Test}
\begin{tcolorbox}[colback=green!5, colframe=green!30!black, title = {\strut}]
Test
\end{tcolorbox}
\end{frame}
\end{document}

Instead of \strut
you could use an empty box: \mbox{}
, or a phantom: \phantom{Ag}
- Here's the result with an empty box:
\documentclass{beamer}
\usepackage{tcolorbox}
\begin{document}
\begin{frame}[t]
\frametitle{Test}
\begin{tcolorbox}[colback=green!5, colframe=green!30!black, title = {\mbox{}}]
Test
\end{tcolorbox}
\end{frame}
\end{document}

Probably there's a much more elegant tcolorbox
solution, but you could always 'freelance' the frame as well:

\documentclass{article}
\usepackage{lipsum}
\usepackage[skins,breakable]{tcolorbox}
\begin{document}
% Example taken from Thomas F. Sturm Manual for version 3.93 p. 346
% Problem: After a break, The style of the boxed title is lost
% Christian Hupfer found a solution that keeps the style of the boxed title after a break in a tcolorbox.
% http://tex.stackexchange.com/questions/315091/tcolorbox-same-boxed-title-style-after-break
% The solution works well for frames made of a continuous line.
% Is there a way to make it work for dashed frames ?
\tcbset{%
boxtitlestuff/.style={%
frame empty,
colback=black!2!white,
},
brokentitle/.style={%
geometry nodes=true,
skin=freelance,
extras first={%
overlay={%
\draw [preaction={draw=green!15!white, line width=1pt, rounded corners}, rounded corners, green!50!white, line width=1pt, dashed] ([yshift=1pt, xshift=-1pt]interior.north west) rectangle ([yshift=-1pt, xshift=1pt]interior.south east);
\node[above, rounded corners, draw, green!50!white, line width=1pt, fill=green!15!white, text=green!50!black, font=\bfseries\normalsize] (A) at ([yshift=-.25\baselineskip]interior.north) {#1};
}
},
extras middle and last={%
overlay={%
\draw [preaction={draw=green!15!white, line width=1pt, rounded corners}, rounded corners,green!50!white, line width=1pt, dashed] ([yshift=1pt, xshift=-1pt]interior.north west) rectangle ([yshift=-1pt, xshift=1pt]interior.south east);
\node[above, rounded corners, draw=black, line width=.8pt, fill=black!2!white, text=black, font=\bfseries\normalsize] (A) at ([yshift=-.25\baselineskip]interior.north) {continued boxed title};
}
}
}
}
\begin{tcolorbox}[%
brokentitle=Boxed title,
breakable,
frame empty,
colframe=green!15!white,
]
\tiny
\lipsum
\lipsum
\end{tcolorbox}
\end{document}
Best Answer
To control the line thickness for the title rule, you can use the
titlerule=<length>
option; unfortunately, in the current version (3.36 when this answers was written) there's no key to individually control the color of the title rule and this line will inherit the frame color; I'm sure Thomas will add this option in a new update :) In the meantime, you can useenhanced
and anoverlay
to control the attributes individually; something along these lines:The output: