The required modifications are very similar to the ones in Biblatex: How to automatically make in-text citations bold given a condition?, but the basis for the cite macro is the definition from alphabetic.cbx and not from numeric.cbx. Furthermore, we need to redefine labelalphawidth for the bibliography and not labelnumberwidth.
\documentclass[british]{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{libertine}
\usepackage{babel}
\usepackage{csquotes}
\usepackage[style=alphabetic, backend=biber]{biblatex}
\usepackage{hyperref}
\DeclareFieldFormat{labelalphawidth}{\mkbibbrackets{\ifkeyword{important}{\mkbibbold{#1}}{#1}}}
\DeclareFieldFormat{boldimportant}{\ifkeyword{important}{\mkbibbold{#1}}{#1}}
\renewbibmacro*{cite}{%
\printtext[bibhyperref]{%
\printtext[boldimportant]{%
\printfield{labelprefix}%
\printfield{labelalpha}%
\printfield{extraalpha}%
\ifbool{bbx:subentry}
{\printfield{entrysetcount}}
{}}}}
\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@inproceedings{appleby,
author = {Humphrey Appleby},
title = {On the Importance of the Civil Service},
booktitle = {Why We Matter},
date = {1980},
keywords = {important},
}
@book{hacker,
author = {James Hacker},
title = {The Government},
date = {1981},
}
\end{filecontents}
\addbibresource{\jobname.bib}
\begin{document}
\cite{appleby,hacker}
\printbibliography
\end{document}
![[**App80**; Hac81], "App80" is in bold.](../../images/b52638a89710d08477c3b8b356fb9376.webp)
biblatexdoes not allow you to mix two different styles in one document. But often there are acceptable workarounds. I suggest you ask a new question about this (ideally with a but of background info). – moewe Sep 29 '19 at 14:10